AIDS is caused by HIV, a virus that originated in African primates and jumped to humans through direct blood contact during the hunting and butchering of wild animals. There are actually two types of HIV with separate animal origins: HIV-1, which causes the vast majority of AIDS cases worldwide, came from chimpanzees in Central Africa, while HIV-2, a less common and less aggressive form, came from a smaller monkey called the sooty mangabey in West Africa.
The Chimpanzee Origin of HIV-1
HIV-1 evolved from a virus called SIV (simian immunodeficiency virus) that naturally infects two subspecies of chimpanzee in Central Africa. These chimps carry the virus without getting sick from it, likely because they’ve coexisted with it for thousands of years. In humans, the virus behaves very differently, attacking the immune system and eventually causing AIDS if untreated.
Molecular clock analysis, a technique that uses the virus’s mutation rate to estimate when it first appeared, places the ancestor of HIV-1 Group M (the strain responsible for the global pandemic) at around 1908, with a range of 1884 to 1924. That means the virus probably crossed from chimpanzees to humans in the early 1900s, decades before anyone recognized the disease.
How the Virus Jumped to Humans
The most widely supported explanation is that hunters in Central Africa were exposed to infected chimpanzee blood while killing or butchering the animals for food, a practice known as bushmeat hunting. The risk of picking up blood-borne viruses is highest during activities involving direct contact with an animal’s blood, such as skinning, cutting, or carrying a fresh kill. A cut or open wound on the hunter’s hand would have been enough to let the virus enter the bloodstream.
This wasn’t a single freak event. Genetic evidence shows that SIV crossed into humans on multiple separate occasions, producing different groups of HIV-1 (groups M, N, O, and P). Group M became the one that spread globally, while the others remained limited to small populations in Central Africa. Each group represents a distinct animal-to-human transmission event.
HIV-2 Came From a Different Primate
HIV-2 has a completely separate origin. It descended from a strain of SIV carried by sooty mangabeys, small monkeys found across West Africa in countries like Liberia, Sierra Leone, and Côte d’Ivoire. The virus likely crossed to humans through the same bushmeat route. HIV-2 is less easily transmitted and progresses to AIDS more slowly than HIV-1, which is one reason it has remained concentrated in West Africa rather than spreading globally.
The Oldest Known Human Cases
The earliest confirmed HIV-1 infection comes from a blood sample collected in 1959 in Léopoldville, the capital of the Belgian Congo (now Kinshasa, Democratic Republic of the Congo). A second sample, taken from a lymph node biopsy of a woman in the same city in 1960, showed that HIV-1 had already diversified into distinct genetic lineages by that point. The fact that the virus was already genetically diverse by 1960 tells researchers it had been circulating in humans for several decades before those samples were taken, consistent with the early 1900s origin estimate.
No one at the time knew what these infections were. AIDS wouldn’t be recognized as a clinical syndrome until 1981, when clusters of unusual infections began appearing in young men in the United States.
How HIV Spread From Africa to the World
For decades, HIV circulated in Central Africa without being detected. The path to a global pandemic traces through Haiti. In the mid-1960s, around 1966, HIV-1 subtype B (the strain that dominates in the Americas and Europe) moved from Africa to Haiti, most likely carried by Haitian professionals returning home after working in the Congo.
Between 1969 and 1972, a single transmission chain carried the virus from Haiti to the United States. Researchers have called this migration the “pandemic clade,” representing a key turning point. From the U.S., subtype B spread to Europe and beyond. By the time doctors in Los Angeles and New York noticed the first AIDS cases in 1981, the virus had been quietly spreading through human populations for roughly 70 years.
Why the Virus Thrived in Humans
SIV has crossed from primates to humans many times throughout history, and most of those spillovers likely went nowhere. A hunter would get infected, perhaps pass it to a few close contacts, and the chain would die out in a small, isolated village. What changed in the early 20th century was urbanization. Colonial-era cities like Léopoldville grew rapidly, concentrating large populations with new transportation networks connecting them to other regions. These conditions gave the virus the human-to-human contact it needed to sustain transmission and eventually reach a global scale.
The combination of a specific historical moment, rapid urban growth in Central Africa, international labor migration, and decades of silent spread before anyone knew the virus existed turned what would have been a dead-end infection into the defining pandemic of the late 20th century.