Since HIV/AIDS was first reported in Thailand in 1984, 1,115,415 adults had been infected as of 2008, with 585,830 having died since 1984. 532,522 Thais were living with HIV/AIDS in 2008. In 2009 the adult prevalence of HIV was 1.3%.CIA World Factbook has listed the 2009 statistics for HIV prevalence by country, and according to these, Thailand has the highest prevalence of HIV in Asia. In 2010 the press reported that Thailand "has the highest HIV prevalence in Southeast Asia".

After Thailand’s first case of HIV/AIDS was reported in 1984, the incidence of infection increased steadily in the country. In 1991, the government adopted a strategy to combat the disease, and in recent years, the number of new infections has declined. However, HIV prevalence had remained the same from 2003 to 2005 (1.4 percent) with more people receiving antiretroviral therapy (ART).

Thailand’s early cases of HIV/AIDS occurred primarily among men who have sex with men (MSM). The virus then spread rapidly to injecting drug users (IDUs), followed by prostitutes. Between 2003 and 2005, there were increases in HIV prevalence from 17 to 28 percent among MSM in Bangkok. In addition, prevalence among IDUs still ranges from 30 to 50 percent. In 2005, more than 40 percent of new infections were among women, the majority of whom were infected through intercourse with long-term lovers. Money and a low level of condom use due to women’s activity in the illegal sex trade are factors responsible for the spread of HIV among this group. Although the prevalence of HIV/AIDS in Thailand has declined, the epidemic has moved to the general population and there is a greater need to match prevention efforts with recent changes in the epidemic.

Thailand ( /ˈtaɪlænd/ TY-land or /ˈtaɪlənd/;Thai: ประเทศไทย, RTGS: Prathet Thai), officially the Kingdom of Thailand (Thai: ราชอาณาจักรไทย, RTGS: Ratcha Anachak Thai; IPA: [râːt.tɕʰā ʔāːnāːtɕàk tʰāj] ( listen)), formerly known as Siam (Thai: สยาม; RTGS: Sayam), is a country located at the centre of the Indochina peninsula in Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and the southern extremity of Burma. Its maritime boundaries include Vietnam in the Gulf of Thailand to the southeast, and Indonesia and India in the Andaman Sea to the southwest.

The country is a constitutional monarchy, headed by King Rama IX, the ninth king of the House of Chakri, who, having reigned since 1946, is the world's longest-serving head of state and the longest-reigning monarch in Thai history. The king of Thailand is titled Head of State, Head of the Armed Forces, the Upholder of the Buddhist religion, and the Defender of all Faiths.

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome (AIDS) is a disease of the human immune system caused by the human immunodeficiency virus (HIV). The illness interferes with the immune system, making people with AIDS much more likely to get infections, including opportunistic infections and tumors that do not affect people with working immune systems. This susceptibility gets worse as the disease continues.

HIV is transmitted in a number of ways including: sexual intercourse (including oral sex and anal sex); contaminated blood transfusions and hypodermic needles; and exchange between mother and baby during pregnancy, childbirth, and breastfeeding. It can be transmitted by any contact of a mucous membrane or the bloodstream with a bodily fluid that has the virus in it, such as the blood, semen, vaginal fluid, preseminal fluid, or breast milk from an infected person.