Human immunodeficiency virus (HIV) is an infection that originated in primates (zoonosis) and was passed into humans.
The main form of HIV is closely related to the simian immunodeficiency virus (SIV) found in chimpanzees in central Africa. It is possible that SIV entered humans through hunting and butchering chimpanzees whereby the hunter was wounded and blood from the chimpanzee entered his blood stream. This is estimated to have happened during the 1920s in Cameroon.
There are actually two types of human immunodeficiency virus (HIV): HIV-1 and HIV-2. HIV-1 is the most widespread and closely related to SIV found in chimpanzees. However, HIV-2 is related to a strain of SIV found in monkeys.
There are several factors that led to the spread of HIV in central Africa in the 1950s. One was the frequent use of injections, often reusing unsterilised needles and syringes, in medical treatment in Kinshasa. Sex work also became more common because of the rampant unemployment in the city. HIV was also evolving, changing and becoming more transmissible.
There is an alternative theory that SIV transferred from chimpanzees to humans through the oral polio vaccine. The vaccine was developed in monkey kidneys and it is supposed that SIV from chimpanzees might have survived in the vaccine preparation process and then transferred to humans. However, SIV or HIV could not be found in the stored oral vaccine samples.
In the 1980s, there were claims circulating that HIV was man-made and was manufactured in a biological weapons laboratory in the United States and transferred to Zaire in 1977. There is no evidence to prove this happened and studies show that HIV-1 was present in Africa and the US way before then. For example, there was a case of a young Black man who died of Kaposi’s sarcoma (an AIDS-defining cancer) in 1968 in the US. Another case was a family in Norway who died of AIDS in 1976 and stored blood samples show that the father had acquired HIV while in Africa in 1961.
This research briefing gives the full overview of the origins of HIV.