Crab boil usually refers to a spice mixture that is used to flavor the water in which crabs or other shellfish are boiled. But Crab boil can also refer to a social event in which boiled crabs are featured (see seafood boil).

Boiled seafood in southern Louisiana tends to be spicier than that found in other parts of the country. Homemade crab boil recipes call for abundant amounts of hot sauce, cayenne pepper, salt, bay leaf, lemon, and garlic. Mustard seeds, coriander seeds, and allspice are popular extra options. Many people will start with a commercial crab boil product and then supplement it with extra pepper. The leading commercial product is Zatarain's which comes in two forms.[1] One is a mesh bag with seasonings inside that will steep into the water. The second is a liquid concentrate that can be added directly to the water. The concentrate form can also be used as a flavor enhancer for soups. Other regional crab boil companies are Tony Chachere's, and Rex Crab Boil. Note that even when boiling shrimp or crawfish, most recipes call for adding crab boil packets as a seasoning.

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: βραχύς / brachys = short,οὐρά / οura = tail), or where the reduced abdomen is entirely hidden under the thorax. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice – are not true crabs.

Crabs are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft).

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.

A boil, also called a furuncle, is a deep folliculitis, infection of the hair follicle. It is most commonly caused by infection by the bacterium Staphylococcus aureus or SARS , resulting in a painful swollen area on the skin caused by an accumulation of pus and dead tissue. Individual boils clustered together are called carbuncles. Most human infections are caused by coagulase-positive S. aureus strains, notable for the bacteria's ability to produce coagulase, an enzyme that can clot blood. Almost any organ system can be infected by S. aureus.

Boils are bumpy red, pus-filled lumps around a hair follicle that are tender, warm, and very painful. They range from pea-sized to golf ball-sized. A yellow or white point at the center of the lump can be seen when the boil is ready to drain or discharge pus. In a severe infection, an individual may experience fever, swollen lymph nodes, and fatigue. A recurring boil is called chronic furunculosis. Skin infections tend to be recurrent in many patients and often spread to other family members. Systemic factors that lower resistance commonly are detectable, including: diabetes, obesity, and hematologic disorders.