A virus is a small infectious agent that can replicate only inside the living cells of organisms. Most viruses are too small to be seen directly with a light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 viruses have been described in detail, although there are millions of different types. Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology.

Virus particles (known as ''virions'') consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium.

The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids – pieces of DNA that can move between cells – while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.

Viruses spread in many ways; viruses in plants are often transmitted from plant to plant by insects that feed on the sap of plants, such as aphids; viruses in animals can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal-oral route and are passed from person to person by contact, entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The range of host cells that a virus can infect is called its "host range". This can be narrow or, as when a virus is capable of infecting many species, broad.

Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. However, some viruses including those causing AIDS and viral hepatitis evade these immune responses and result in chronic infections. Antibiotics have no effect on viruses, but several antiviral drugs have been developed.

Etymology

The word is from the Latin ''virus'' referring to poison and other noxious substances, first used in English in 1392. ''Virulent'', from Latin ''virulentus'' (poisonous), dates to 1400. A meaning of "agent that causes infectious disease" is first recorded in 1728, before the discovery of viruses by Dmitry Ivanovsky in 1892. The plural is ''viruses''. The adjective ''viral'' dates to 1948. The term ''virion'' is also used to refer to a single infective viral particle.

History

Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected using a microscope. In 1884, the French microbiologist Charles Chamberland invented a filter (known today as the Chamberland filter or Chamberland-Pasteur filter) with pores smaller than bacteria. Thus, he could pass a solution containing bacteria through the filter and completely remove them from the solution. In 1892, the Russian biologist Dmitry Ivanovsky used this filter to study what is now known as the tobacco mosaic virus. His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration. Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea. At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium – this was part of the germ theory of disease. In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that the filtered solution contained a new form of infectious agent. He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a ''contagium vivum fluidum'' (soluble living germ) and re-introduced the word ''virus''. Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanley, who proved they were particulate. In the same year Friedrich Loeffler and Frosch passed the first animal virus – agent of foot-and-mouth disease (aphthovirus) – through a similar filter.

In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages (or commonly ''phages''), and the French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria on agar, would produce areas of dead bacteria. He accurately diluted a suspension of these viruses and discovered that the highest dilutions (lowest virus concentrations), rather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension. Phages were heralded as a potential treatment for diseases such as typhoid and cholera, but their promise was forgotten with the development of penicillin. The study of phages provided insights into the switching on and off of genes, and a useful mechanism for introducing foreign genes into bacteria.

By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to be filtered, and their requirement for living hosts. Viruses had been grown only in plants and animals. In 1906, Ross Granville Harrison invented a method for growing tissue in lymph, and, in 1913, E. Steinhardt, C. Israeli, and R. A. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue. In 1928, H. B. Maitland and M. C. Maitland grew vaccinia virus in suspensions of minced hens' kidneys. Their method was not widely adopted until the 1950s, when poliovirus was grown on a large scale for vaccine production.

Another breakthrough came in 1931, when the American pathologist Ernest William Goodpasture grew influenza and several other viruses in fertilized chickens' eggs. In 1949, John F. Enders, Thomas Weller, and Frederick Robbins grew polio virus in cultured human embryo cells, the first virus to be grown without using solid animal tissue or eggs. This work enabled Jonas Salk to make an effective polio vaccine.

The first images of viruses were obtained upon the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll. In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it was mostly made of protein. A short time later, this virus was separated into protein and RNA parts. The tobacco mosaic virus was the first to be crystallised and its structure could therefore be elucidated in detail. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in 1941. On the basis of her pictures, Rosalind Franklin discovered the full DNA structure of the virus in 1955. In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells.

The second half of the 20th century was the golden age of virus discovery and most of the 2,000 recognised species of animal, plant, and bacterial viruses were discovered during these years. In 1957, equine arterivirus and the cause of Bovine virus diarrhea (a pestivirus) were discovered. In 1963, the hepatitis B virus was discovered by Baruch Blumberg, and in 1965, Howard Temin described the first retrovirus. Reverse transcriptase, the key enzyme that retroviruses use to translate their RNA into DNA, was first described in 1970, independently by Howard Martin Temin and David Baltimore. In 1983 Luc Montagnier's team at the Pasteur Institute in France, first isolated the retrovirus now called HIV.

Origins

Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose. There are three main hypotheses that try to explain the origins of viruses:

; Regressive hypothesis : Viruses may have once been small cells that parasitised larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell. This is also called the ''degeneracy hypothesis'', or ''reduction hypothesis''. ; Cellular origin hypothesis : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids (pieces of naked DNA that can move ''between'' cells) or transposons (molecules of DNA that replicate and move around to different positions ''within'' the genes of the cell). Once called "jumping genes", transposons are examples of mobile genetic elements and could be the origin of some viruses. They were discovered in maize by Barbara McClintock in 1950. This is sometimes called the ''vagrancy hypothesis'', or the ''escape hypothesis''.

; Coevolution hypothesis : This is also called the ''virus-first hypothesis'' and proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on earth and would have been dependent on cellular life for billions of years. Viroids are molecules of RNA that are not classified as viruses because they lack a protein coat. However, they have characteristics that are common to several viruses and are often called subviral agents. Viroids are important pathogens of plants. They do not code for proteins but interact with the host cell and use the host machinery for their replication. The hepatitis delta virus of humans has an RNA genome similar to viroids but has a protein coat derived from hepatitis B virus and cannot produce one of its own. It is, therefore, a defective virus and cannot replicate without the help of hepatitis B virus. In similar manner, the virophage 'sputnik' is dependent on mimivirus, which infects the protozoan ''Acanthamoeba castellanii''. These viruses that are dependent on the presence of other virus species in the host cell are called ''satellites'' and may represent evolutionary intermediates of viroids and viruses.

In the past, there were problems with all of these hypotheses: the regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis contravened the definition of viruses in that they require host cells. Viruses are now recognised as ancient and to have origins that pre-date the divergence of life into the three domains. This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses.

The evidence for an ancestral world of RNA cells and computer analysis of viral and host DNA sequences are giving a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses. To date, such analyses have not proved which of these hypotheses is correct. However, it seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms.

Prions are infectious protein molecules that do not contain DNA or RNA. They cause an infection in sheep called scrapie and cattle bovine spongiform encephalopathy ("mad cow" disease). In humans they cause kuru and Creutzfeldt-Jakob disease. They are able to replicate because some proteins can exist in two different shapes and the prion changes the normal shape of a host protein into the prion shape. This starts a chain reaction where each prion protein converts many host proteins into more prions, and these new prions then go on to convert even more protein into prions. Although they are fundamentally different from viruses and viroids, their discovery gives credence to the idea that viruses could have evolved from self-replicating molecules.

Microbiology

Life properties

Opinions differ on whether viruses are a form of life, or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes and evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism, and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell – although bacterial species such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.

Structure

Viruses display a wide diversity of shapes and sizes, called ''morphologies''. Generally viruses are much smaller than bacteria. Most viruses that have been studied have a diameter between 20 and 300 nanometres. Some filoviruses have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses cannot be seen with a light microscope so scanning and transmission electron microscopes are used to visualise virions. To increase the contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals, such as tungsten, that scatter the electrons from regions covered with the stain. When virions are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only.

A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. These are formed from identical protein subunits called capsomeres. Viruses can have a lipid "envelope" derived from the host cell membrane. The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy. In general, there are four main morphological virus types:

; Helical: These viruses are composed of a single type of capsomer stacked around a central axis to form a helical structure, which may have a central cavity, or hollow tube. This arrangement results in rod-shaped or filamentous virions: These can be short and highly rigid, or long and very flexible. The genetic material, in general, single-stranded RNA, but ssDNA in some cases, is bound into the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of capsomers. The well-studied tobacco mosaic virus is an example of a helical virus.

; Icosahedral: Most animal viruses are icosahedral or near-spherical with icosahedral symmetry. A regular icosahedron is the optimum way of forming a closed shell from identical sub-units. The minimum number of identical capsomers required is twelve, each composed of five identical sub-units. Many viruses, such as rotavirus, have more than twelve capsomers and appear spherical but they retain this symmetry. Capsomers at the apices are surrounded by five other capsomers and are called pentons. Capsomers on the triangular faces are surrounded by six others and are called hexons.

; Envelope: Some species of virus envelop themselves in a modified form of one of the cell membranes, either the outer membrane surrounding an infected host cell or internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a viral envelope. This membrane is studded with proteins coded for by the viral genome and host genome; the lipid membrane itself and any carbohydrates present originate entirely from the host. The influenza virus and HIV use this strategy. Most enveloped viruses are dependent on the envelope for their infectivity.

; Complex: These viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall. Some bacteriophages, such as Enterobacteria phage T4, have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibres. This tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell.

The poxviruses are large, complex viruses that have an unusual morphology. The viral genome is associated with proteins within a central disk structure known as a nucleoid. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole virion is slightly pleiomorphic, ranging from ovoid to brick shape. Mimivirus is the largest known virus, with a capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral.

Some viruses that infect Archaea have complex structures that are unrelated to any other form of virus, with a wide variety of unusual shapes, ranging from spindle-shaped structures, to viruses that resemble hooked rods, teardrops or even bottles. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures.

Genome

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DNA

RNA

Both DNA and RNA (at different stages in the life cycle)

|- |Shape||

Linear

Circular

Segmented

|- |Strandedness||

Single-stranded

Double-stranded

Double-stranded with regions of single-strandedness

|- |Sense||

Positive sense (+)

Negative sense (−)

Ambisense (+/−)

|}

An enormous variety of genomic structures can be seen among viral species; as a group they contain more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses,

Viral genomes are circular, as in the polyomaviruses, or linear, as in the adenoviruses. The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses and certain DNA viruses, the genome is often divided up into separate parts, in which case it is called ''segmented''. For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid. However, all segments are not required to be in the same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses.

A viral genome, irrespective of nucleic acid type, is almost always either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to a ladder. The virus particles of some virus families, such as those belonging to the ''Hepadnaviridae'', contain a genome that is partially double-stranded and partially single-stranded.

For most viruses with RNA genomes and some with single-stranded DNA genomes, the single strands are said to be either positive-sense (called the plus-strand) or negative-sense (called the minus-strand), depending on whether or not they are complementary to the viral messenger RNA (mRNA). Positive-sense viral RNA is in the same sense as viral mRNA and thus at least a part of it can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. DNA nomenclature for viruses with single-sense genomic ssDNA is similar to RNA nomenclature, in that the ''coding strand'' for the viral mRNA is complementary to it (−), and the ''non-coding strand'' is a copy of it (+). However, several types of ssDNA and ssRNA viruses have genomes that are ambisense in that transcription can occur off both strands in a double-stranded replicative intermediate. Examples include geminiviruses, which are ssDNA plant viruses and arenaviruses, which are ssRNA viruses of animals.

Genome size varies greatly between species. The smallest viral genomes – the ssDNA circoviruses, family ''Circoviridae'' – code for only two proteins and have a genome size of only 2 kilobases; the largest – mimiviruses – have genome sizes of over 1.2 megabases and code for over one thousand proteins. RNA viruses generally have smaller genome sizes than DNA viruses because of a higher error-rate when replicating, and have a maximum upper size limit. Beyond this limit, errors in the genome when replicating render the virus useless or uncompetitive. To compensate for this, RNA viruses often have segmented genomes – the genome is split into smaller molecules – thus reducing the chance that an error in a single-component genome will incapacitate the entire genome. In contrast, DNA viruses generally have larger genomes because of the high fidelity of their replication enzymes. Single-strand DNA viruses are an exception to this rule, however, as mutation rates for these genomes can approach the extreme of the ssRNA virus case.

Viruses undergo genetic change by several mechanisms. These include a process called genetic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are "silent" – they do not change the protein that the gene encodes – but others can confer evolutionary advantages such as resistance to antiviral drugs. Antigenic shift occurs when there is a major change in the genome of the virus. This can be a result of recombination or reassortment. When this happens with influenza viruses, pandemics might result. RNA viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are a prime target for natural selection.

Segmented genomes confer evolutionary advantages; different strains of a virus with a segmented genome can shuffle and combine genes and produce progeny viruses or (offspring) that have unique characteristics. This is called reassortment or ''viral sex''.

Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied. Recombination is common to both RNA and DNA viruses.

Replication cycle

Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they ''assemble'' in the cell.

The life cycle of viruses differs greatly between species but there are six ''basic'' stages in the life cycle of viruses:

''Attachment'' is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, HIV infects a limited range of human leucocytes. This is because its surface protein, gp120, specifically interacts with the CD4 molecule – a chemokine receptor – which is most commonly found on the surface of CD4+ T-Cells. This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope protein to undergo changes that results in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow the virus to enter.

''Penetration'' follows attachment: Virions enter the host cell through receptor-mediated endocytosis or membrane fusion. This is often called viral entry. The infection of plant and, it is presumed, fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose, and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall. However, nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in the form of single-stranded nucleoprotein complexes, through pores called plasmodesmata. This process requires movement proteins, which are virus-encoded proteins probably originally derived from plant proteins, which interact with the plasmodesmatal transport machinery Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. However, given that bacterial cell walls are much less thick than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while the viral capsid remains outside.

''Uncoating'' is a process in which the viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid.

''Replication'' of viruses primarily involves multiplication of the genome. Replication involves synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive sense RNA viruses), viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins.

Following the structure-mediated self-''assembly'' of the virus particles, some modification of the proteins often occurs. In viruses such as HIV, this modification (sometimes called maturation) occurs ''after'' the virus has been released from the host cell. Viruses can be ''released'' from the host cell by lysis, a process that kills the cell by bursting its membrane and cell wall if present: This is a feature of many bacterial and some animal viruses. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination into a specific place in the host's chromosome. The viral genome is then known as a "provirus" or, in the case of bacteriophages a "prophage". Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host; however, at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding. During this process the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane.

The genetic material within virus particles, and the method by which the material is replicated, varies considerably between different types of viruses.

; DNA viruses : The genome replication of most DNA viruses takes place in the cell's nucleus. If the cell has the appropriate receptor on its surface, these viruses enter the cell sometimes by direct fusion with the cell membrane (e.g. herpesviruses) or – more usually – by receptor-mediated endocytosis. Most DNA viruses are entirely dependent on the host cell's DNA and RNA synthesising machinery, and RNA processing machinery; however, viruses with larger genomes may encode much of this machinery themselves. In eukaryotes the viral genome must cross the cell's nuclear membrane to access this machinery, while in bacteria it need only enter the cell.

; RNA viruses: Replication usually takes place in the cytoplasm. RNA viruses can be placed into four different groups depending on their modes of replication. The polarity (whether or not it can be used directly by ribosomes to make proteins) of single-stranded RNA viruses largely determines the replicative mechanism; the other major criterion is whether the genetic material is single-stranded or double-stranded. All RNA viruses use their own RNA replicase enzymes to create copies of their genomes.

; Reverse transcribing viruses: These have ssRNA (''Retroviridae'', ''Metaviridae'', ''Pseudoviridae'') or dsDNA (''Caulimoviridae'', and ''Hepadnaviridae'') in their particles. Reverse transcribing viruses with RNA genomes (retroviruses), use a DNA intermediate to replicate, whereas those with DNA genomes (pararetroviruses) use an RNA intermediate during genome replication. Both types use a reverse transcriptase, or RNA-dependent DNA polymerase enzyme, to carry out the nucleic acid conversion. Retroviruses integrate the DNA produced by reverse transcription into the host genome as a provirus as a part of the replication process; pararetroviruses do not, although integrated genome copies of especially plant pararetroviruses can give rise to infectious virus. They are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme, e.g. zidovudine and lamivudine. An example of the first type is HIV, which is a retrovirus. Examples of the second type are the ''Hepadnaviridae'', which includes Hepatitis B virus.

Effects on the host cell

The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called ''cytopathic effects''. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis. Often cell death is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle.

Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses. Some viruses, such as Epstein-Barr virus, can cause cells to proliferate without causing malignancy, while others, such as papillomaviruses, are established causes of cancer.

Host range

Viruses are by far the most abundant parasites on earth, and they have been found to infect all types of cellular life including animals, plants, and bacteria. However, different types of viruses can infect only a limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species – in this case humans, and are said to have a narrow host range. Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing.

Classification

Classification seeks to describe the diversity of viruses by naming and grouping them on the basis of similarities. In 1962, André Lwoff, Robert Horne, and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system. This system bases classification on phylum, class, order, family, genus, and species. Viruses were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes. Later the International Committee on Taxonomy of Viruses was formed. However, viruses are not classified on the basis of phylum or class, as their small genome size and high rate of mutation makes it difficult to determine their ancestry beyond Order. As such, the Baltimore Classification is used to supplement the more traditional hierarchy.

ICTV classification

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and wrote guidelines that put a greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. The 7th lCTV Report formalised for the first time the concept of the virus species as the lowest taxon (group) in a branching hierarchy of viral taxa. However, at present only a small part of the total diversity of viruses has been studied, with analyses of samples from humans finding that about 20% of the virus sequences recovered have not been seen before, and samples from the environment, such as from seawater and ocean sediments, finding that the large majority of sequences are completely novel.

The general taxonomic structure is as follows:

:Order (-virales) ::Family (-viridae) :::Subfamily (-virinae) ::::Genus (''-virus'') :::::Species (''-virus'')

In the current (2008) ICTV taxonomy, five orders have been established, the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, and Picornavirales. The committee does not formally distinguish between subspecies, strains, and isolates. In total there are 5 orders, 82 families, 11 subfamilies, 307 genera, 2,083 species and about 3,000 types yet unclassified.

Baltimore classification

The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification.

The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). Additionally, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups:

As an example of viral classification, the chicken pox virus, varicella zoster (VZV), belongs to the order Herpesvirales, family ''Herpesviridae'', subfamily ''Alphaherpesvirinae'', and genus ''Varicellovirus''. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase.

Role in human disease

Examples of common human diseases caused by viruses include the common cold, influenza, chickenpox and cold sores. Many serious diseases such as ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between human herpes virus six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is controversy over whether the borna virus, previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans.

Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the viral species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die, the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which causes cold sores, to remain in a dormant state within the human body. This is called latency and is a characteristic of the herpes viruses including Epstein-Barr virus, which causes glandular fever, and varicella zoster virus, which causes chickenpox and shingles. Most people have been infected with at least one of these types of herpes virus. However, these latent viruses might sometimes be beneficial, as the presence of the virus can increase immunity against bacterial pathogens, such as ''Yersinia pestis''.

Some viruses can cause life-long or chronic infections, where the viruses continue to replicate in the body despite the host's defence mechanisms. This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected are known as carriers, as they serve as reservoirs of infectious virus. In populations with a high proportion of carriers, the disease is said to be endemic.

Epidemiology

Viral epidemiology is the branch of medical science that deals with the transmission and control of virus infections in humans. Transmission of viruses can be vertical, that is from mother to child, or horizontal, which means from person to person. Examples of vertical transmission include hepatitis B virus and HIV where the baby is born already infected with the virus. Another, more rare, example is the varicella zoster virus, which, although causing relatively mild infections in humans, can be fatal to the foetus and new-born baby.

Horizontal transmission is the most common mechanism of spread of viruses in populations. Transmission can occur when: body fluids are exchanged during sexual activity, e.g., HIV; blood is exchanged by contaminated transfusion or needle sharing, e.g., hepatitis C; a child is born to an infected mother, e.g., hepatitis B; exchange of saliva by mouth, e.g., Epstein-Barr virus; contaminated food or water is ingested, e.g., norovirus; aerosols containing virions are inhaled, e.g., influenza virus; and insect vectors such as mosquitoes penetrate the skin of a host, e.g., dengue. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e., those not immune), the quality of healthcare and the weather.

Epidemiology is used to break the chain of infection in populations during outbreaks of viral diseases. Control measures are used that are based on knowledge of how the virus is transmitted. It is important to find the source, or sources, of the outbreak and to identify the virus. Once the virus has been identified, the chain of transmission can sometimes be broken by vaccines. When vaccines are not available sanitation and disinfection can be effective. Often infected people are isolated from the rest of the community and those that have been exposed to the virus placed in quarantine. To control the outbreak of foot and mouth disease in cattle in Britain in 2001, thousands of cattle were slaughtered. Most viral infections of humans and other animals have incubation periods during which the infection causes no signs or symptoms. Incubation periods for viral diseases range from a few days to weeks but are known for most infections. Somewhat overlapping, but mainly following the incubation period, there is a period of communicability; a time when an infected individual or animal is contagious and can infect another person or animal. This too is known for many viral infections and knowledge the length of both periods is important in the control of outbreaks. When outbreaks cause an unusually high proportion of cases in a population, community or region they are called epidemics. If outbreaks spread worldwide they are called pandemics.

Epidemics and pandemics

Native American populations were devastated by contagious diseases, in particular, smallpox, brought to the Americas by European colonists. It is unclear how many Native Americans were killed by foreign diseases after the arrival of Columbus in the Americas, but the numbers have been estimated to be close to 70% of the indigenous population. The damage done by this disease significantly aided European attempts to displace and conquer the native population.

A pandemic is a worldwide epidemic. The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly influenza A virus. The victims were often healthy young adults, in contrast to most influenza outbreaks, which predominantly affect juvenile, elderly, or otherwise-weakened patients.

The Spanish flu pandemic lasted from 1918 to 1919. Older estimates say it killed 40–50 million people, while more recent research suggests that it may have killed as many as 100 million people, or 5% of the world's population in 1918. Most researchers believe that HIV originated in sub-Saharan Africa during the 20th century; it is now a pandemic, with an estimated 38.6 million people now living with the disease worldwide. The Joint United Nations Programme on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognised on June 5, 1981, making it one of the most destructive epidemics in recorded history. In 2007 there were 2.7 million new HIV infections and 2 million HIV-related deaths.

Several highly lethal viral pathogens are members of the ''Filoviridae''. Filoviruses are filament-like viruses that cause viral hemorrhagic fever, and include the ebola and marburg viruses. The Marburg virus attracted widespread press attention in April 2005 for an outbreak in Angola. Beginning in October 2004 and continuing into 2005, the outbreak was the world's worst epidemic of any kind of viral hemorrhagic fever.

Cancer

Viruses are an established cause of cancer in humans and other species. Viral cancers occur only in a minority of infected persons (or animals). Cancer viruses come from a range of virus families, including both RNA and DNA viruses, and so there is no single type of "oncovirus" (an obsolete term originally used for acutely transforming retroviruses). The development of cancer is determined by a variety of factors such as host immunity and mutations in the host. Viruses accepted to cause human cancers include some genotypes of human papillomavirus, hepatitis B virus, hepatitis C virus, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus and human T-lymphotropic virus. The most recently discovered human cancer virus is a polyomavirus (Merkel cell polyomavirus) that causes most cases of a rare form of skin cancer called Merkel cell carcinoma. Hepatitis viruses can develop into a chronic viral infection that leads to liver cancer. Infection by human T-lymphotropic virus can lead to tropical spastic paraparesis and adult T-cell leukemia. Human papillomaviruses are an established cause of cancers of cervix, skin, anus, and penis. Within the ''Herpesviridae'', Kaposi's sarcoma-associated herpesvirus causes Kaposi's sarcoma and body cavity lymphoma, and Epstein–Barr virus causes Burkitt's lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disorder, and nasopharyngeal carcinoma. Merkel cell polyomavirus closely related to SV40 and mouse polyomaviruses that have been used as animal models for cancer viruses for over 50 years.

Host defence mechanisms

The body's first line of defence against viruses is the innate immune system. This comprises cells and other mechanisms that defend the host from infection in a non-specific manner. This means that the cells of the innate system recognise, and respond to, pathogens in a generic way, but, unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host.

RNA interference is an important innate defence against viruses. Many viruses have a replication strategy that involves double-stranded RNA (dsRNA). When such a virus infects a cell, it releases its RNA molecule or molecules, which immediately bind to a protein complex called dicer that cuts the RNA into smaller pieces. A biochemical pathway called the RISC complex is activated, which degrades the viral mRNA and the cell survives the infection. Rotaviruses avoid this mechanism by not uncoating fully inside the cell and by releasing newly produced mRNA through pores in the particle's inner capsid. The genomic dsRNA remains protected inside the core of the virion.

When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies that bind to the virus and render it non-infectious. This is called humoral immunity. Two types of antibodies are important. The first, called IgM, is highly effective at neutralizing viruses but is produced by the cells of the immune system only for a few weeks. The second, called IgG, is produced indefinitely. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past. IgG antibody is measured when tests for immunity are carried out.

A second defence of vertebrates against viruses is called cell-mediated immunity and involves immune cells known as T cells. The body's cells constantly display short fragments of their proteins on the cell's surface, and, if a T cell recognises a suspicious viral fragment there, the host cell is destroyed by ''killer T'' cells and the virus-specific T-cells proliferate. Cells such as the macrophage are specialists at this antigen presentation. The production of interferon is an important host defence mechanism. This is a hormone produced by the body when viruses are present. Its role in immunity is complex; it eventually stops the viruses from reproducing by killing the infected cell and its close neighbours.

Not all virus infections produce a protective immune response in this way. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. These persistent viruses evade immune control by sequestration, blockade of antigen presentation, cytokine resistance, evasion of natural killer cell activities, escape from apoptosis, and antigenic shift. Other viruses, called ''neurotropic viruses'', are disseminated by neural spread where the immune system may be unable to reach them.

Prevention and treatment

Because viruses use vital metabolic pathways within host cells to replicate, they are difficult to eliminate without using drugs that cause toxic effects to host cells in general. The most effective medical approaches to viral diseases are vaccinations to provide immunity to infection, and antiviral drugs that selectively interfere with viral replication.

Vaccines

Vaccination is a cheap and effective way of preventing infections by viruses. Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella. Smallpox infections have been eradicated. Vaccines are available to prevent over thirteen viral infections of humans, and more are used to prevent viral infections of animals. Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens). Live vaccines contain weakened forms of the virus, which do not cause the disease but, nonetheless, confer immunity. Such viruses are called attenuated. Live vaccines can be dangerous when given to people with a weak immunity, (who are described as immunocompromised), because in these people, the weakened virus can cause the original disease. Biotechnology and genetic engineering techniques are used to produce subunit vaccines. These vaccines use only the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine. Subunit vaccines are safe for immunocompromised patients because they cannot cause the disease. The yellow fever virus vaccine, a live-attenuated strain called 17D, is probably the safest and most effective vaccine ever generated.

Antiviral drugs

Antiviral drugs are often nucleoside analogues, (fake DNA building-blocks), which viruses mistakenly incorporate into their genomes during replication. The life-cycle of the virus is then halted because the newly synthesised DNA is inactive. This is because these analogues lack the hydroxyl groups, which, along with phosphorus atoms, link together to form the strong "backbone" of the DNA molecule. This is called DNA chain termination. Examples of nucleoside analogues are aciclovir for Herpes simplex virus infections and lamivudine for HIV and Hepatitis B virus infections. Aciclovir is one of the oldest and most frequently prescribed antiviral drugs. Other antiviral drugs in use target different stages of the viral life cycle. HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. There is a large class of drugs called protease inhibitors that inactivate this enzyme.

Hepatitis C is caused by an RNA virus. In 80% of people infected, the disease is chronic, and without treatment, they are infected for the remainder of their lives. However, there is now an effective treatment that uses the nucleoside analogue drug ribavirin combined with interferon. The treatment of chronic carriers of the hepatitis B virus by using a similar strategy using lamivudine has been developed.

Infection in other species

Viruses infect all cellular life and, although viruses occur universally, each cellular species has its own specific range that often infect only that species. Some viruses, called satellites, can only replicate within cells that have already been infected by another virus. Viruses are important pathogens of livestock. Diseases such as Foot and Mouth Disease and bluetongue are caused by viruses. Companion animals such as cats, dogs, and horses, if not vaccinated, are susceptible to serious viral infections. Canine parvovirus is caused by a small DNA virus and infections are often fatal in pups. Like all invertebrates, the honey bee is susceptible to many viral infections. Fortunately, most viruses co-exist harmlessly in their host and cause no signs or symptoms of disease.

Plants

There are many types of plant virus, but often they cause only a loss of yield, and it is not economically viable to try to control them. Plant viruses are often spread from plant to plant by organisms, known as ''vectors''. These are normally insects, but some fungi, nematode worms, and single-celled organisms have been shown to be vectors. When control of plant virus infections is considered economical, for perennial fruits, for example, efforts are concentrated on killing the vectors and removing alternate hosts such as weeds. Plant viruses are harmless to humans and other animals because they can reproduce only in living plant cells.

Plants have elaborate and effective defence mechanisms against viruses. One of the most effective is the presence of so-called resistance (R) genes. Each R gene confers resistance to a particular virus by triggering localised areas of cell death around the infected cell, which can often be seen with the unaided eye as large spots. This stops the infection from spreading. RNA interference is also an effective defence in plants. When they are infected, plants often produce natural disinfectants that kill viruses, such as salicylic acid, nitric oxide, and reactive oxygen molecules.

Plant virus particles or virus-like particles (VLPs) have applications in both biotechnology and nanotechnology. The capsids of most plant viruses are simple and robust structures and can be produced in large quantities either by the infection of plants or by expression in a variety of heterologous systems. Plant virus particles can be modified genetically and chemically to encapsulate foreign material and can be incorporated into supramolecular structures for use in biotechnology.

Bacteria

Bacteriophages are a common and diverse group of viruses and are the most abundant form of biological entity in aquatic environments – there are up to ten times more of these viruses in the oceans than there are bacteria, reaching levels of 250,000,000 bacteriophages per millilitre of seawater. These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins, which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and, in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released.

The major way bacteria defend themselves from bacteriophages is by producing enzymes that destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells. Bacteria also contain a system that uses CRISPR sequences to retain fragments of the genomes of viruses that the bacteria have come into contact with in the past, which allows them to block the virus's replication through a form of RNA interference. This genetic system provides bacteria with acquired immunity to infection.

Archaea

Some viruses replicate within archaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes. These viruses have been studied in most detail in the thermophilic archaea, particularly the orders Sulfolobales and Thermoproteales. Defences against these viruses may involve RNA interference from repetitive DNA sequences within archaean genomes that are related to the genes of the viruses.

Role in aquatic ecosystems

Viruses are the most abundant biological entity in aquatic environments: a teaspoon of seawater contains about one million of them. They are essential to the regulation of saltwater and freshwater ecosystems. Most of these viruses are bacteriophages, which are harmless to plants and animals. They infect and destroy the bacteria in aquatic microbial communities, comprising the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the bacterial cells by the viruses stimulates fresh bacterial and algal growth.

Microorganisms constitute more than 90% of the biomass in the sea. It is estimated that viruses kill approximately 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms, which often kill other marine life. The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.

The effects of marine viruses are far-reaching; by increasing the amount of photosynthesis in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 gigatonnes of carbon per year.

Like any organism, marine mammals are susceptible to viral infections. In 1988 and 2002, thousands of harbour seals were killed in Europe by phocine distemper virus. Many other viruses, including caliciviruses, herpesviruses, adenoviruses and parvoviruses, circulate in marine mammal populations.

Role in evolution

Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution. It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes and at the time of the last universal common ancestor of life on Earth. Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.

Applications

Life sciences and medicine

Viruses are important to the study of molecular and cellular biology as they provide simple systems that can be used to manipulate and investigate the functions of cells. The study and use of viruses have provided valuable information about aspects of cell biology. For example, viruses have been useful in the study of genetics and helped our understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.

Geneticists often use viruses as vectors to introduce genes into cells that they are studying. This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. It shows promising use in the treatment of cancer and in gene therapy. Eastern European scientists have used phage therapy as an alternative to antibiotics for some time, and interest in this approach is increasing, because of the high level of antibiotic resistance now found in some pathogenic bacteria.

Expression of heterologous proteins by viruses is the basis of several manufacturing processes that are currently being used for the production of various proteins such as vaccine antigens and antibodies. Industrial processes have been recently developed using viral vectors and a number of pharmaceutical proteins are currently in pre-clinical and clinical trials.

Materials science and nanotechnology

Current trends in nanotechnology promise to make much more versatile use of viruses. From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles. Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.

Because of their size, shape, and well-defined chemical structures, viruses have been used as templates for organizing materials on the nanoscale. Recent examples include work at the Naval Research Laboratory in Washington, DC, using Cowpea Mosaic Virus (CPMV) particles to amplify signals in DNA microarray based sensors. In this application, the virus particles separate the fluorescent dyes used for signalling to prevent the formation of non-fluorescent dimers that act as quenchers. Another example is the use of CPMV as a nanoscale breadboard for molecular electronics.

Synthetic viruses

Many viruses can be synthesized de novo (“from scratch”) and the first synthetic virus was created in 2002. Although somewhat of a misconception, it is not the actual virus that is synthesized, but rather its DNA genome (in case of a DNA virus), or a cDNA copy of its genome (in case of RNA viruses). For many virus families the naked synthetic DNA or RNA (once enzymatically converted back from the synthetic cDNA) is infectious when introduced into a cell. That is, they contain all the necessary information to produce new viruses. This technology is now being used to investigate novel vaccine strategies. The ability to synthesize viruses has far-reaching consequences, since viruses can no longer be regarded as extinct, as long as the information of their genome sequence is known and permissive cells are available. Currently, the full-length genome sequences of 2408 different viruses (including smallpox) are publicly available at an online database, maintained by the National Institute of Health.

Weapons

The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponised for biological warfare. Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory. The smallpox virus devastated numerous societies throughout history before its eradication. There are officially only two centers in the world that keep stocks of smallpox virus – the Russian Vector laboratory, and the United States Centers for Disease Control. But fears that it may be used as a weapon are not totally unfounded; the vaccine for smallpox has sometimes severe side-effects – during the last years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox – and smallpox vaccination is no longer universally practiced. Thus, much of the modern human population has almost no established resistance to smallpox.

References

Notes

Bibliography

Collier, Leslie; Balows, Albert; Sussman, Max (1998) ''Topley and Wilson's Microbiology and Microbial Infections'' ninth edition, Volume 1, ''Virology'', volume editors: Mahy, Brian and Collier, Leslie. Arnold. ISBN 0-340-66316-2.

Dimmock, N.J; Easton, Andrew J; Leppard, Keith (2007) ''Introduction to Modern Virology'' sixth edition, Blackwell Publishing, ISBN 1-4051-3645-6.

Knipe, David M; Howley, Peter M; Griffin, Diane E; Lamb, Robert A; Martin, Malcolm A; Roizman, Bernard; Straus Stephen E. (2007) ''Fields Virology'', Lippincott Williams & Wilkins. ISBN 0-7817-6060-7.

Shors, Teri (2008). ''Understanding Viruses''. Jones and Bartlett Publishers. ISBN 0-7637-2932-9.

External links

ViralZone A Swiss Institute of Bioinformatics resource for all viral families, providing general molecular and epidemiological information

"A Gazillion Tiny Avatars", article on viruses by Olivia Judson, NY Times, Dec 15, 2009

''Viruses'' – an Open Access journal

* Viruses

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name	Jennifer Aniston
birth name	Jennifer Joanna Aniston
birth date	February 11, 1969
birth place	Sherman Oaks, California
occupation	Actress
years active	1989–present
spouse	Brad Pitt (2000–05)
parents	John Aniston, Nancy Dow }}

Jennifer Joanna Aniston (born February 11, 1969) is an American actress, film director, and producer. She gained worldwide recognition in the 1990s for portraying Rachel Green in the television sitcom ''Friends'', a role which earned her an Emmy Award, a Golden Globe Award, and a Screen Actors Guild Award.

Aniston has also enjoyed a successful Hollywood film career. She gained critical acclaim for her performances in the independent films ''She's the One'' (1996), ''Office Space'' (1999), ''The Good Girl'' (2002) and ''Friends with Money'' (2006). She has had her greatest commercial successes with the films ''Bruce Almighty'' (2003), ''The Break-Up'' (2006), ''Marley & Me'' (2008), ''Just Go with It'' (2011), and ''Horrible Bosses'' (2011). Aniston has been selected for a Star on the Hollywood Walk of Fame in 2012.

Early life

Jennifer Aniston was born in Sherman Oaks, Los Angeles to actors John Aniston and Nancy Dow. Her father is Greek and a native of Crete. Her mother was born in New York City of Scottish-Canadian and Italian descent. Aniston has two half-brothers, John Melick, her maternal older half-brother, and Alex Aniston, her younger paternal half-brother. Aniston's godfather was actor Telly Savalas, one of her father's best friends. As a child, Aniston lived in Greece for one year with her family. They later relocated to New York City. Aniston attended the Rudolf Steiner School in New York, and graduated from Manhattan's Fiorello H. LaGuardia High School of Music & Art and Performing Arts. She worked in Off Broadway productions such as ''For Dear Life'' and ''Dancing on Checker's Grave'', and supported herself with several part-time jobs, which included working as a telemarketer, waitress, and bike messenger. In 1989, Aniston moved to Los Angeles, California.

Career

Television

Aniston was cast in her first television role in 1990, starring as a regular on the short-lived series ''Molloy'' and in the TV movie ''Camp Cucamonga''. She also co-starred in ''Ferris Bueller'', a television adaptation of the 1986 hit movie ''Ferris Bueller's Day Off''. The series, however, was quickly canceled. Aniston then appeared in two more failed television comedy series, ''The Edge'' and ''Muddling Through'', and guest-starred on ''Quantum Leap'', ''Herman's Head'', and ''Burke's Law''. After the string of canceled shows, along with her appearance in the critically derided 1992 horror film, ''Leprechaun'', Aniston reportedly considered giving up acting.

Aniston's plans changed, however, after auditioning for ''Friends'', a sitcom that was set to debut on NBC's 1994–1995 fall lineup. The producers of the show originally wanted Aniston to audition for the role of Monica Geller, but Courteney Cox was considered to be better suited to the role. Thus, Aniston was cast as Rachel Green. She was also offered a spot as a featured player on ''Saturday Night Live'', but turned it down to do ''Friends''. She played the character of Rachel from 1994 until the show ended in 2004.

The program was successful and Aniston, along with her co-stars, gained world-wide reputation among television viewers. Aniston received a salary of $1 million per episode for the last two seasons of ''Friends'', as well as five Emmy nominations (two for Supporting Actress, three for Lead Actress), including a win for Outstanding Lead Actress in a Comedy Series. According to the ''Guinness World Book of Records'' (2005), Aniston (along with her female costars) became the highest paid TV actress of all time with her $1 million-per-episode paycheck for the tenth season of ''Friends''.

In 2007, Aniston guest starred in an episode of Courteney Cox Arquette's series ''Dirt'', playing Arquette's rival, Tina Harrod. She appeared in the third episode of Season 3 of NBC's ''30 Rock'' playing Liz Lemon's old college roommate who stalks Jack Donaghy. On July 16, 2009, Aniston received an Emmy nomination in the category of Outstanding Guest Actress in a Comedy Series for her role on ''30 Rock''. Aniston was a guest star on the Season 2 premiere of ABC's sitcom ''Cougar Town'', playing a psychiatrist.

Film

While appearing on ''Friends'', Aniston also started segueing into film roles. She made her film debut in 1992's ''Leprechaun'', which was negatively reviewed by critics, but was commercially successful and gained a cult following. Following a four year hiatus from cinema, Aniston returned to film work in 1996, when she played supporting roles in the independent films ''Dream for an Insomniac'', and ''She's the One'', alongside Edward Burns and Cameron Diaz. Aniston's first starring vehicle was the film ''Picture Perfect'' (1997), in which she starred opposite Kevin Bacon and Jay Mohr. While the film received mixed reviews, Aniston's performance was more warmly received, with many critics suggesting that she had screen presence. During the second half of the 1990s she starred in several films such as the ensemble romantic film '''Til There Was You'' (1997) with Dylan McDermott and Sarah Jessica Parker, ''The Thin Pink Line'' (1998), the animated feature ''The Iron Giant'' (1999), and the critically acclaimed comedy ''Office Space'' (1999). She gained critical acclaim for her performances in ''The Object of My Affection'' (1998), a comedy-drama about a woman who falls for a gay man (played by Paul Rudd), and in the low-budget 2002 film ''The Good Girl'', playing an unglamorous cashier in a small town. The latter film opened in relatively few theaters—under 700 in total—taking $14M in the U.S. box office.

Aniston's biggest box office success to date was her appearance in 2003's ''Bruce Almighty'', in which she played the live-in girlfriend of Jim Carrey's title character. Aniston then starred in the 2004 film ''Along Came Polly'' opposite Ben Stiller. In late 2005, Aniston appeared in two major studio films, ''Derailed'' and ''Rumor Has It…''. In 2006, Aniston appeared in the low-budget drama ''Friends with Money'', which was first shown at the Sundance Film Festival and received a limited release. Aniston's next film, ''The Break-Up'', which was released on June 2, grossed approximately $39.17 million during its opening weekend, despite lukewarm reviews. In 2006, Aniston directed a hospital emergency room-set short film called ''Room 10'', starring Robin Wright Penn and Kris Kristofferson as part of ''Glamour'''s Reel Moments film series. Aniston noted that she was inspired to direct by actress Gwyneth Paltrow, who also directed a short film in 2006.

On December 25, 2008, ''Marley & Me'', in which Aniston starred alongside Owen Wilson, was released. It set a record for the largest Christmas Day box office ever with $14.75 million in ticket sales. It earned a total of $51.7 million over the four-day weekend and placed #1 at the box office, a position it maintained for two weeks. The total worldwide gross was $242,717,113. Her next film in wide release, ''He's Just Not That into You'', where she starred opposite Ben Affleck, opened in February 2009. The movie made $27.5 million, ranking #1 at the box office in its opening weekend. While the film received mixed reviews, Aniston, along with Affleck, Ginnifer Goodwin, and Jennifer Connelly, were often praised by critics as being the stand-outs in the film.

In March 2010, Aniston appeared in ''The Bounty Hunter'', which costarred Gerard Butler. While the film received scathing reviews from critics, it was a modest box office success, garnering over $130 million worldwide. A lukewarm box-office reception greeted her next film, ''The Switch'', in which she co-starred with Jason Bateman. The film's opening weekend drew what ''The Hollywood Reporter'' dubbed "a dispiriting $8.4 million". The film has received generally mixed reviews, with review site Metacritic showing 13 out of 30 critics delivering a positive verdict.

As of June 20, 2010, Aniston's movies have grossed more than $1 billion in the United States and over $1.7 billion worldwide. Aniston's ''Just Go with It'', with Adam Sandler, was released on Valentine's Day weekend in 2011. The story is about a plastic surgeon, played by Sandler, who asks his office manager, played by Aniston, to pose as his wife, to prove his honesty to his much younger girlfriend, played by Brooklyn Decker.

Also in 2011, Aniston starred in the comedy movie ''Horrible Bosses'', with Colin Farrell, Jason Bateman, Charlie Day, and Jamie Foxx, directed by Seth Gordon. The film focuses on a trio of employees who plot to murder their titular tyrannical supervisors. Aniston played one of the bosses, a sexually aggressive dentist who harasses Charlie Day's character. Aniston has signed to star in ''Wanderlust'' with Paul Rudd, who she co-starred with in the 1998 movie ''The Object of My Affection'' and also ''Friends''. The script, which has been bought by Universal Pictures, was written by Rudd, Ken Marino, and David Wain with the latter also directing the film and will be produced by Judd Apatow. The movie tells the story of a married couple who join a commune after losing their money and deciding modern life is not for them.

Other work

Aniston has appeared in various commercials and music videos. In 1996, she was in Tom Petty and The Heartbreakers music video for "Walls". In 2001, Aniston was in Melissa Etheridge's music video for "I Want To Be In Love". She was cast in a Heineken commercial which was later banned due to branding issues. Aniston has also been in commercials for L'Oreal hair products. In 1994, Microsoft invited Aniston, along with ''Friends'' co-star Matthew Perry, to shoot a 30-minute promotional advertisement for their new operating system, Windows 95. Along with Brad Pitt and Brad Grey, CEO of Paramount Pictures, Aniston founded the film production company Plan B Entertainment in 2002, although she and Grey withdrew in 2005. In 2008, she and partner Kristin Hahn formed the production company Echo Films.

Aniston worked for over a year on a new perfume, which was released on July 21, 2010, at Harrods in London. Original plans called for the perfume to be named ''Lolavie by Jennifer Aniston'', but to avoid confusion with a similarly named perfume, the name was changed to simply ''Jennifer Aniston''. In an interview following the launch, Aniston said that she would also like to create a fragrance for men.

Since 2007, Aniston has worked in a publicity campaign for the drink SmartWater; on March 7, 2011, she released a YouTube video, titled "Jennifer Aniston Goes Viral", for SmartWater, tripling online interest in the product within 24 hours of the release.

Philanthropy

Aniston is a supporter of Friends of El Faro, a grassroots non-profit organization that helps raise money for Casa Hogar Sion, an orphanage in Tijuana, Mexico. She has appeared in many TV commercials for St. Jude's Children's Research Hospital, which she supports. She also hosted September 2008's Stand Up to Cancer show.

In the "It Can't Wait" campaign to free Burma, Aniston directed and starred in a video.

On April 14, 2007, Aniston received GLAAD's Vanguard Award for her contributions to increased visibility and understanding of the lesbian, gay, bisexual and transgender community. On Earth Day 2010, she joined Courteney Cox, Woody Harrelson, Ben Stiller and others in "The Cove PSA: My Friend is..." an effort to stop the slaughter of dolphins and protect the Japanese people from the toxic levels of mercury found in dolphin meat. Other charities that Aniston has supported include AmeriCares, Clothes Off Our Back, Feeding America, EB Medical Research Foundation, Project A.L.S., OmniPeace, and Rape, Abuse & Incest National Network.

Aniston donated $500,000 to Doctors Without Borders, Haitian health care provider Partners in Health and AmeriCares, and also participated in the megastar-studded Hope for Haiti Now telethon.

Personal life

Aniston dated her ''Ferris Bueller'' TV costar Charlie Schlatter in 1990, and briefly dated musician Adam Duritz in 1995. From 1995 to 1998, she was romantically involved with actor Tate Donovan and the couple were reportedly engaged.

In May 1998, she began dating actor Brad Pitt. They married on July 29, 2000, in a private wedding ceremony in Malibu. For a few years, their marriage was considered the rare Hollywood success. However, the couple announced their separation on January 6, 2005. Aniston, however, filed for divorce on March 25, 2005. It was finalized on October 2, 2005. During this period there was intense speculation in the media that Pitt had been unfaithful to Aniston with his ''Mr. & Mrs. Smith'' co-star, Angelina Jolie.

In the following months, the public's reaction towards the divorce was reported in the press. The story became the headline news of media shows such as ''Entertainment Tonight'' and ''Access Hollywood'', and made the front-pages of tabloid magazines for years, continuing till this day. 'Team Aniston' and 'Team Jolie' T-shirts appeared, with 'Team Aniston' shirts out-selling the 'Team Jolie' shirts 4:1. Aniston revealed that her divorce prompted her to reach out to her mother, Nancy, from whom she was estranged for nearly a decade. They initially became estranged when Nancy spoke about her daughter on a television show and later wrote a book entitled, ''From Mother and Daughter to Friends: A Memoir'' (1999). Aniston has also stated she was devastated by the death of her longtime therapist, whose work helped make her separation from Pitt easier. Aniston said her relationship with Pitt, which she does not regret, was "seven very intense years together" and that "it was a beautiful, complicated relationship."

After her divorce, Aniston began a relationship with actor Vince Vaughn, with whom she costarred in ''The Break-Up''. Relationship troubles were reported in September 2006, followed by a confirmed split in December of that year. In February 2008, she began dating singer John Mayer.

Aniston has had two septoplasties to correct her deviated septum—one which was incorrectly done in 1994 and the second in January 2007.

Aniston is the godmother of Coco Riley Arquette, the daughter of her good friends, actors Courteney Cox and David Arquette.

In 2007, ''Forbes'' rated Aniston as one of the top 10 richest women in entertainment and estimated her net worth to be about $110 million.

In the media

''Forbes'' listed Aniston as the 10th richest woman in the entertainment industry for 2007. Aniston was also included in the annual Star Salary Top 10 of trade magazine ''The Hollywood Reporter'' for 2006. According to ''Forbes'', in October 2007, Aniston was the top-selling celebrity face of the entertainment industry. She was also Hollywood's most profitable actress. Aniston has been on the ''Forbes'' Celebrity 100 list, based on "earnings and fame", every year since 2001, topping the list in 2003. For the year of 2008, ''Forbes'' listed Aniston's earnings as $27 million. In 2005, Aniston became the first-ever GQ Woman of the Year. She has appeared on ''People''s annual list of The Most Beautiful every year since 1995, and came in at #1 in 2004. In 2006, Aniston also topped ''People''s Best Dressed List. She has been a regular on FHM's 100 Sexiest Women list since 1996, ranking at #81 in 2010, #24 in 2009 and #27 in 2008. In 2011 ''The Telegraph'' reported the most sought after body parts of the rich and famous revealed by two Hollywood plastic surgeons who carried out a survey among their patients to build up the picture of what the perfect woman would look like. Under the category of the most sought after body shape Aniston was voted in the top three alongside Gisele Bundchen and Penelope Cruz. Although Aniston disliked the hairstyle she wore during her first two years on ''Friends'', "The Rachel" became and remains very popular among women.

Salary

Friends

Season One: $22,500 per episode.

Season Two: $38,000 per episode

Season Three: $75,000 per episode

Season Four: $85,000 per episode

Season Five: $100,000 per episode

Season Six: $125,000 per episode

Season Seven: $750,000 per episode

Season Eight: $750,000 per episode

Season Nine: $1,000,000 per episode

Season Ten: $1,000,000 per episode

Film

Picture Perfect (1997): $2,000,000

Rock Star (2001): $3,000,000

Along Came Polly (2004): $5,000,000

Rumour Has It... (2005): $8,000,000

The Break-Up (2006): $8,000,000

Marley & Me (2008): $8,000,000

The Bounty Hunter (2010): $8,000,000

The Switch (2010): $8,000,000

Just Go with It (2011): $10,000,000

Horrible Bosses (2011): $6,000,000

Wanderlust (2011): $8,000,000

Filmography

+ Film
Year	Title	Role	Notes
1993		Tory Reding
1996	''She's the One''	Renee Fitzpatrick
1996	''Dream for an Insomniac''	Allison
1997	'''Til There Was You''	Debbie
1997		Kate Mosley
1998	''''	Clove
1998	''Waiting for Woody''	Herself	Short film
1998	''The Object of My Affection''	Nina Borowski
1999	''Office Space''	Joanna	Cult hit
1999	''''	Annie Hughes	Voice only
2001		Emily Poule
2002	''''	Justine Last	Independent film
2003	''Bruce Almighty''	Grace Connelly
2003		Herself	Cameo appearance
2004	''Along Came Polly''	Polly Prince
2005		Lucinda Harris
2005	''Rumor Has It...''	Sarah Huttinger
2006	''Friends with Money''	Olivia	Independent film
2006	''''	Brooke Meyers
2008		Jenny Grogan
2009		Beth Murphy
2009		Sue Claussen	Independent film. Also Executive Producer
2009	''Love Happens''	Eloise Chandler
2009	''Journey to Sundance''	Herself	Documentary
2010	''''	Nicole Hurly
2010	''''	Kassie Larson	Also Executive Producer
2011	''Just Go with It''	Katherine Murphy
2011	''Horrible Bosses''	Dr. Julia Harris
2011		Linda	Post-production

+ Television
Year	Title	Role	Notes
1990	''Molloy''	Courtney	Main role
1990	''Camp Cucamonga''	Ava Schector	Television movie
1990–1991		Jeannie Bueller
1992–1993	''''	Various characters	Main role
1994	''Muddling Through''	Madeline Drego Cooper	Main role
1994–2004	''Friends''	Rachel Green	236 episodes; one of the six main roles
2004		Herself – Hostess	Documentary

+ Television guest appearances
Year	Title	Role	Notes
1992		Kiki Wilson	"Nowhere to Run" (Season 5, Episode 4)
1992–1993	''Herman's Head''	Suzie Brooks	"Twisted Sister" (Season 1, Episode 25) "Jay Is for Jealousy" (Season 3, Episode 8)
1994	''Burke's Law''	Linda Campbell	"Who Killed the Beauty Queen?" (Season 1, Episode 4)
1995	''The Larry Sanders Show''	Herself	"Conflict of Interest" (Season 4, Episode 10)
1998		CPA Suzanne	"Follow the Clams?" (Season 1, Episode 17)
1998		Herself	"Ellen: A Hollywood Tribute: Part 2" (Season 5, Episode 20)
1998	''Disney's Hercules''	Galatea (Voice)	"Dream Date" (Season 1, Episode 27)
1999	''South Park''	Mrs. Stevens – Choir Teacher (Voice)	"Rainforest Shmainforest" (Season 3, Episode 1)
2003	''Freedom: A History of Us''	Jessie Benton	"Wake Up America " (Season 1, Episode 4)
2003	''King of the Hill''	Pepperoni Sue/Stephanie (Voice)
2007		Tina Harrod	"Ita Missa Est" (Season 1, Episode 13)
2008	''30 Rock''	Claire Harper	"The One with the Cast of Night Court" (Season 3, Episode 3)
2010	''Cougar Town''	Glenn
2011	''Kickin' It''	Laura	Recurring role (3 episodes)

+ Video games
Year	Title	Role	Notes
1996	''Steven Spielberg's Director's Chair''	Laura	Voice

Director

Year	Title	Notes
2006	''Room 10''	Short Film
2012	''Project Five''	TV Movie

Producer

Year	Title	Notes
2008		Executive producer
2010		Executive producer
2011	''The Goree Girls''	Producer

Awards and nominations

Aniston has won multiple awards during her career, and has been recognized for her work in both television and movie productions.

Year	Award	Category	Title of work	Result
1996	American Comedy Awards	Funniest Supporting Female Performer in a TV Series	Friends
1996	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
1997	Kid's Choice Awards	Favorite Television Actress	Friends
1999	American Comedy Awards	Funniest Supporting Female Performer in a TV Series	Friends
1999	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
1999	Kid's Choice Awards	Favorite Television Actress	Friends
2000	Emmy Awards	Outstanding Supporting Actress in a Comedy Series	Friends
2000	Satellite Awards	Best Performance by an Actress in a Series, Comedy or Musical	Friends
2000	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
2000	TV Guide Awards	Editor's Choice
2000	Kid's Choice Awards	Favorite Television Actress	Friends
2001	American Comedy Awards	Funniest Supporting Female Performer in a TV Series	Friends
2001	Emmy Awards	Outstanding Supporting Actress in a Comedy Series	Friends
2001	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
2001	People's Choice Awards	Favorite Female Television Performer	Friends
2001	Aftonbladet TV Prize, Sweden	Best Foreign TV Personality – Female	Friends
2002	Emmy Awards	Outstanding Lead Actress in a Comedy Series	Friends
2002	Golden Globe Awards	Best Performance by an Actress in a Supporting Role in a Series, Mini-Series or Motion Picture Made for Television	Friends
2002	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
2002	Screen Actors Guild Awards	Outstanding Performance by a Female Actor in a Comedy Series	Friends
2002	People's Choice Awards	Favorite Female Television Performer	Friends
2002	Hollywood Film Festival	Actress of the Year
2002	Teen Choice Awards	Choice TV Actress – Comedy	Friends
2002	Aftonbladet TV Prize, Sweden	Best Foreign TV Personality – Female	Friends
2002	Kid's Choice Awards	Favorite Television Actress	Friends
2003	Emmy Awards	Outstanding Lead Actress in a Comedy Series	Friends
2003	Golden Globe Awards	Best Performance by an Actress in a Television Series – Musical or Comedy	Friends
2003	Satellite Awards	Best Performance by an Actress in a Series, Comedy or Musical	Friends
2003	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
2003	Screen Actors Guild Awards	Outstanding Performance by a Female Actor in a Comedy Series	Friends
2003	Independent Spirit Awards	Best Female Lead	The Good Girl
2003	Satellite Awards	Best Performance by an Actress in a Motion Picture, Comedy or Musical	The Good Girl
2003	Online Film Critics Society	Best Actress	The Good Girl
2003	Teen Choice Awards	Choice Movie Actress – Drama/Action Adventure	The Good Girl
2003	Teen Choice Awards	Choice Movie Liplock	The Good Girl
2003	Teen Choice Awards	Choice Movie Liar	The Good Girl
2003	Teen Choice Awards	Choice Movie Actress – Comedy	Bruce Almighty
2003	Teen Choice Awards	Choice TV Actress – Comedy	Friends
2003	People's Choice Awards	Favorite Female Television Performer	Friends
2003	Aftonbladet TV Prize, Sweden	Best Foreign TV Personality – Female	Friends
2003	Logie Awards	Most Popular Overseas TV Program	Friends
2003	Kid's Choice Awards	Favorite Television Actress	Friends
2004	Emmy Awards	Outstanding Lead Actress in a Comedy Series	Friends
2004	Screen Actors Guild Awards	Outstanding Performance by an Ensemble in a Comedy Series	Friends
2004	Logie Awards	Most Popular Overseas Star	Friends
2004	Logie Awards	Most Popular Overseas TV Program	Friends
2004	MTV Movie Awards	Best Kiss	Bruce Almighty
2004	MTV Movie Awards	Best Dance Sequence	Along Came Polly
2004	People's Choice Awards	Favorite Female Television Performer	Friends
2004	Teen Choice Awards	Choice TV Actress – Comedy	Friends
2004	Aftonbladet TV Prize, Sweden	Best Foreign TV Personality – Female	Friends
2004	Kid's Choice Awards	Favorite Television Actress	Friends
2005	ShoWest Convention Awards	Female Star of the Year
2005	TV Land Awards	Little Screen/Big Screen Star
2006	TV Land Awards	Most Memorable Kiss	Friends
2006	TV Land Awards	Little Screen/Big Screen Star
2006	Teen Choice Awards	Choice Movie Chemistry (shared with Vince Vaughn)	The Break-Up
2006	Teen Choice Awards	Choice Movie Actress – Comedy	The Break-Up
2007	People's Choice Awards	Favorite Female Movie Star
2007	People's Choice Awards	Favorite On-Screen Match-Up	The Break-Up
2007	TV Land Awards	Little Screen/Big Screen Star
2007	TV Land Awards	Break Up That Was So Bad It Was Good	Friends
2007	GLAAD Media Awards	Vanguard Award
2007	CineVegas International Film Festival	Best Short Film	Room 10
2009	Emmy Awards	Outstanding Guest Actress in a Comedy Series	30 Rock
2009
2009	Teen Choice Awards	Choice Movie Actress – Comedy	Marley & Me
2009	Teen Choice Awards	Choice Movie Actress – Comedy	He's Just Not That Into You
2009	Kid's Choice Awards	Favorite Movie Actress	Marley & Me
2010	People's Choice Awards	Favorite Movie Actress
2011	MTV Movie Awards	Best Female Performance	Just Go With It
2011	Spike Guys' Choice Awards	Decade of Hotness

References

External links

Category:1969 births Category:20th-century actors Category:21st-century actors Category:Actors from Los Angeles, California Category:Actors from New York City Category:American people of Greek descent Category:American people of Italian descent Category:American people of Scottish descent Category:American film actors Category:American television actors Category:American voice actors Category:Best Musical or Comedy Actress Golden Globe (television) winners Category:Emmy Award winners Category:English-language film directors Category:Female film directors Category:Film directors from California Category:Film directors from New York City Category:film producers from California Category:Fiorello H. LaGuardia High School alumni Category:GLAAD Media Awards winners Category:Living people Category:Outstanding Performance by an Ensemble in a Comedy Series Screen Actors Guild Award winners Category:People from Hollywood Category:People from Manhattan Category:People from Sherman Oaks, Los Angeles Category:Waldorf school alumni

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