There is now a wealth of evidence on how, when and where HIV first began to cause illness in humans. You can find out more about the origins and history of HIV through our interactive timeline , where you can read, watch, listen and explore key events from the history of the epidemic.
The researchers who discovered this connection concluded that it proved chimpanzees were the source of HIV-1, and that the virus had at some point crossed species from chimps to humans.
They discovered that the chimps had hunted and eaten two smaller species of monkeys red-capped mangabeys and greater spot-nosed monkeys. These smaller monkeys infected the chimps with two different strains of SIV. The two different SIV strains then joined together to form a third virus SIVcpz that could be passed on to other chimps.
This is the strain that can also infect humans. The most commonly accepted theory is that of the 'hunter'. In this scenario, SIVcpz was transferred to humans as a result of chimps being killed and eaten, or their blood getting into cuts or wounds on people in the course of hunting.
This supports the hunter theory because every time SIV passed from a chimpanzee to a human, it would have developed in a slightly different way within the human body, and produced a slightly different strain. This explains why there is more than one strain of HIV How did HIV-2 get passed to humans? It is far rarer, and less infectious than HIV As a result, it infects far fewer people, and is mainly found in a few countries in West Africa like Mali, Mauritania, Nigeria and Sierra Leone.
Studies of some of the earliest known samples of HIV provide clues about when it first appeared in humans and how it evolved. The first verified case of HIV is from a blood sample taken in from a man living in what is now Kinshasa in the Democratic Republic of Congo.
The sample was retrospectively analysed and HIV detected. There are numerous earlier cases where patterns of deaths from common opportunistic infections, now known to be AIDS-defining, suggest that HIV was the cause, but this is the earliest incident where a blood sample can verify infection.
Many of the first cases of AIDS were recorded there too. How did HIV spread from Kinshasa? The area around Kinshasa is full of transport links, such as roads, railways and rivers. The area also had a growing sex trade around the time that HIV began to spread.
The high population of migrants and sex trade might explain how HIV spread along these infrastructure routes. By , it had reached Brazzaville, about km west of Kinshasa. The lack of transport routes into the North and East of the country accounts for the significantly fewer reports of infections there at the time.
At this time, many Haitian professionals who were working in the colonial Democratic Republic of Congo during the s returned to Haiti. By , this subtype had caused 75 million infections.
In , a few cases of rare diseases were being reported among gay men in New York and California, such as Kaposi's Sarcoma a rare cancer and a lung infection called PCP. At first the disease was called all sorts of names relating to the word 'gay'. What is the 'Four-H-Club'?