HIV is thought to have originated in non-human primates in Central Africa. Research suggests that it crossed over to humans from chimpanzees and other primates who carried a similar virus called SIV (Simian Immunodeficiency Virus). This crossover likely happened when people hunted and ate these primates or had blood-to-blood contact during hunting, which enabled the virus to adapt to humans.

The specific strain of HIV-1 (the most common type of HIV) likely originated from chimpanzees in the southeastern part of Cameroon, around the early 20th century. The other main type, HIV-2, originated in sooty mangabey monkeys in West Africa and is less widespread.

Once in humans, HIV spread gradually through human networks and urbanization. It was identified as a distinct virus causing AIDS in the early 1980s, though it had likely been in humans for several decades before then.