HIV has been infecting humans for roughly a century. The strain responsible for the global pandemic, known as HIV-1 group M, began spreading in people around the early 1900s in central Africa. But the virus wasn’t identified until 1983, and the disease it causes wasn’t formally recognized until 1981, meaning it circulated undetected for decades before anyone knew it existed.
The Jump From Animals to Humans
HIV didn’t start as a human virus. It evolved from a closely related virus called SIV (simian immunodeficiency virus) that infects primates. HIV-1, the type responsible for the vast majority of infections worldwide, came from chimpanzees in southeastern Cameroon. HIV-2, a less common and generally less aggressive type found mainly in West Africa, came from a smaller monkey species called sooty mangabeys.
These weren’t single events. HIV-1 actually crossed into humans at least four separate times, producing four distinct lineages labeled groups M, N, O, and P. Group M is the one that became a global pandemic. The other groups remained relatively confined. Scientists believe the initial transmission likely happened through hunting and butchering of primates, which would have exposed people to infected blood.
Molecular clock analysis, a technique that uses the rate of genetic mutations to estimate when viruses diverged, places the common ancestor of the chimpanzee virus at around 1492, meaning the primate version of the virus is centuries old. But the specific jump that produced HIV-1 group M in humans is estimated to have occurred in the late 1800s or very early 1900s. HIV-2 subtypes A and B similarly crossed into humans during the first half of the twentieth century, with its rapid spread in Guinea-Bissau not beginning until the 1960s and 1970s.
Kinshasa: Where the Pandemic Took Root
After crossing into humans in Cameroon, the virus most likely traveled south along river trade routes to what is now Kinshasa, the capital of the Democratic Republic of the Congo. During the German colonization of Cameroon (1884 to 1916), frequent ferry traffic along the Sangha River system connected southern Cameroon to Kinshasa, driven by the rubber and ivory trade. This created a pathway for the virus to reach a growing urban center.
Kinshasa became the epicenter. A major 2014 phylogenetic study found that the city has the highest probability of being the pandemic’s geographic origin, and that 57% of all early viral lineage movements originated from there. Today, Kinshasa still exhibits more HIV-1 genetic diversity than anywhere else on Earth, a fingerprint of its long history with the virus. The epidemic grew slowly at first, roughly tracking population growth, until around 1960. After that, social upheaval, urbanization, and expanding transport networks triggered an epidemiological transition, and group M began spreading far faster than the population was growing.
The Oldest Physical Evidence
The oldest confirmed HIV-1 sample comes from 1959. Designated ZR59, it was recovered from a blood specimen collected in what was then Léopoldville (now Kinshasa). A second early sample came from a lymph node biopsy taken in 1960 from an adult woman in the same city. When researchers compared the genetic sequences of these two samples, they found they were already substantially different from each other, proving the virus had been diversifying in the human population for decades before either sample was collected.
Outside Africa, the virus arrived earlier than most people assume. An analysis of preserved blood samples from Haitian AIDS patients treated in Miami in the early 1980s revealed that HIV had reached Haiti by the mid-1960s. From there, it entered the United States around 1969, roughly twelve years before doctors in Los Angeles noticed the first cluster of unusual illnesses.
When AIDS Was First Recognized
On June 5, 1981, the CDC published a short report in its weekly bulletin describing five young men in Los Angeles who had been treated for a rare lung infection called Pneumocystis carinii pneumonia between October 1980 and May 1981. Two had already died. All five had severe immune deficiency that couldn’t be explained by any known condition. This report is considered the formal beginning of the recognized AIDS epidemic, though neither the disease nor its cause had a name yet.
Cases quickly surfaced in other cities. Similar clusters of rare infections and cancers appeared in New York and San Francisco, and doctors began recognizing a pattern of catastrophic immune system failure in otherwise healthy young adults.
Identifying the Virus
The cause of AIDS remained a mystery for nearly two years. In December 1982, a team at the Institut Pasteur in Paris, led by virologists Luc Montagnier and Françoise Barré-Sinoussi with researcher Jean-Claude Chermann, began examining lymph node tissue from a patient. Within about two weeks, they detected a new retrovirus. The first photograph of the virus was taken on February 4, 1983, and the discovery was published in the journal Science on May 20, 1983.
The virus was initially called Lymphadenopathy-Associated Virus, or LAV. A competing American team led by Robert Gallo at the National Institutes of Health published similar findings in 1984, calling the virus HTLV-III. In 1986, the international scientific community settled the naming dispute and agreed to call it HIV: Human Immunodeficiency Virus.
The Scale of the Epidemic Today
What began as a localized infection in a handful of people in central Africa has become one of the most significant infectious disease events in human history. As of the end of 2024, approximately 40.8 million people are living with HIV worldwide. The virus has been found in virtually every country, though sub-Saharan Africa continues to bear the heaviest burden.
The timeline from origin to pandemic spans roughly a century of undetected spread, followed by four decades of intense scientific effort. The virus circulated silently for at least 60 years before medicine recognized the disease it causes, and another two years passed before the virus itself was identified. That long silent period, shaped by colonial trade routes, urbanization, and war, is what allowed a single cross-species event in a Cameroonian forest to become a global crisis.