HIV first crossed from chimpanzees into humans around the 1920s in what is now southeastern Cameroon. The virus circulated quietly in central Africa for decades before doctors in Los Angeles noticed a strange cluster of infections in 1981, marking the official start of the AIDS epidemic. But the story of how a primate virus became a global pandemic stretches back much further than that first clinical report.
The Jump From Primates to Humans
HIV did not appear out of nowhere. It evolved from a closely related virus called SIV (simian immunodeficiency virus) that naturally infects wild chimpanzees in west central Africa. The specific subspecies responsible, the central chimpanzee, carries a strain of SIV that is the direct ancestor of HIV-1, the type responsible for the vast majority of infections worldwide.
Genetic detective work has traced the geographic source to a remote area in the southeast corner of Cameroon. Hunters who killed and butchered chimpanzees for food likely exposed themselves to infected blood through cuts or wounds, giving the virus a route into the human body. This didn’t happen just once. Researchers have identified at least four separate cross-species jumps from chimpanzees (and possibly gorillas) to humans, each producing a distinct group of HIV-1: groups M, N, O, and P. Group M is the one that spread globally and accounts for roughly 95% of all HIV infections.
There is also a second, less common type of HIV. HIV-2, found mostly in West Africa, jumped to humans from a different primate entirely: the sooty mangabey, a smaller monkey native to the region. Molecular clock estimates place that transmission during the 1940s, roughly two decades after the main HIV-1 jump.
The 1920s: A Virus Takes Root
By analyzing the genetic differences between modern HIV strains and working backward, researchers have calculated that the common ancestor of HIV-1 group M existed sometime around the 1920s, give or take about 15 years. This means the virus was already circulating in at least one human by that point, even though no one at the time had any idea it existed.
After crossing into humans in Cameroon, the virus made its way roughly 700 kilometers south to Kinshasa (then called Léopoldville), the capital of what is now the Democratic Republic of Congo. This bustling colonial city, with its growing population, trade routes, and river transportation networks, provided the conditions the virus needed to spread and diversify. By the mid-20th century, HIV-1 group M had already split into the major subtypes that exist today, all while remaining completely invisible to medicine.
The Oldest Physical Evidence
The earliest confirmed HIV-positive human samples come from Kinshasa. Short fragments of viral genetic material have been recovered from archived blood plasma and tissue specimens dating to 1959 and 1960. These fragments belong to two different subtypes, which tells researchers that the virus had already been diversifying in the human population for decades by that point.
In a remarkable feat of molecular archaeology, scientists later recovered a near-complete HIV genome from a preserved tissue sample taken in Kinshasa in 1966. That specimen, found among 1,645 archived tissue blocks from the late 1950s and 1960s, remains the oldest near-full-length HIV genome ever recovered. Only two of those 1,645 samples tested positive for HIV, illustrating just how rare the virus still was in the general population even decades after its initial jump.
How HIV Reached the Americas
HIV traveled from central Africa to the Caribbean before making its way to North America. Phylogenetic analysis, which reconstructs the virus’s family tree using genetic data, pinpoints the arrival of HIV in the United States to approximately 1970. The ancestral U.S. virus landed in New York City, which became the critical hub from which the epidemic radiated across the continent.
From New York, HIV spread to San Francisco and other cities in California during the 1970s, silently infecting people for roughly a decade before anyone recognized the disease it caused. As University of Arizona virologist Michael Worobey has noted, the genetic signal of the virus’s emergence in the U.S. is clear: “Right around 1970, we see the signal of emergence of this virus, which is evidence that it had to have been there at that point in time in at least one person.”
1981: The World Finally Notices
On June 4, 1981, the CDC published a brief report in its weekly bulletin describing five young men in Los Angeles who had been treated for a rare type of pneumonia between October 1980 and May 1981. All five were previously healthy, and two had already died. The infection, Pneumocystis pneumonia, almost never occurred in people with functioning immune systems. Something was destroying these patients’ ability to fight off disease.
That short bulletin was the first published report of what would, a year later, be named acquired immunodeficiency syndrome, or AIDS. It alerted the medical and public health communities a full four months before the first peer-reviewed journal article on the subject appeared. Within weeks, similar clusters of unusual infections and rare cancers were being reported in New York, San Francisco, and other cities. The virus itself would not be identified until 1983, and it would not be named HIV until 1986.
A Timeline Spanning Decades
The gap between the virus’s actual origin and its discovery is striking. HIV had been infecting humans for roughly 60 years before anyone knew it existed. It had diversified into multiple subtypes, spread across continents, and established itself in major cities on both sides of the Atlantic, all without triggering a recognizable alarm. Several factors explain this long silent period: the virus can take years to cause symptoms, the earliest cases occurred in regions with limited medical surveillance, and the infections it caused (pneumonia, tuberculosis, wasting diseases) mimicked other common illnesses in tropical settings.
The colonial-era transportation networks of central Africa, particularly the river routes connecting Cameroon to Kinshasa, played a key role in the virus’s early geographic spread. Urbanization concentrated enough people in one place for sustained transmission. And by the time global travel carried HIV to the Caribbean and then to the United States, it had already been quietly evolving for half a century.