AIDS was first reported June 5, 1981, when the U.S. Centers for Disease Control and Prevention recorded a cluster of Pneumocystis carinii pneumonia (now classified as Pneumocystis jiroveci pneumonia) in five homosexual men in Los Angeles.Originally dubbed GRID, or Gay-Related Immune Deficiency, health authorities soon realized that nearly half of the people identified with the syndrome were not homosexual men. In 1982, the CDC introduced the term AIDS to describe the newly recognized syndrome.

Three of the earliest known instances of HIV infection are as follows:

A plasma sample taken in 1959 from an adult male living in what is now the Democratic Republic of the Congo.

HIV found in tissue samples from a 15 year old African-American teenager who died in St. Louis in 1969.

HIV found in tissue samples from a Norwegian sailor who died around 1976.

Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is more virulent and more easily transmitted. HIV-1 is the source of the majority of HIV infections throughout the world, while HIV-2 is not as easily transmitted and is largely confined to West Africa. Both HIV-1 and HIV-2 are of primate origin. The origin of HIV-1 is the Central Common Chimpanzee (Pan troglodytes troglodytes) found in southern Cameroon.It is established that HIV-2 originated from the Sooty Mangabey (Cercocebus atys), an Old World monkey of Guinea Bissau, Gabon, and Cameroon.

Most experts believe that HIV probably transferred to humans as a result of direct contact with primates, for instance during hunting or butchery.[118] A more controversial theory known as the OPV AIDS hypothesis suggests that the AIDS epidemic was inadvertently started in the late 1950s in the Belgian Congo by Hilary Koprowski's research into a polio vaccine. According to scientific consensus, this scenario is not supported by the available evidence.