Where did HIV come from?

Current research indicates that HIV-1 came from Pan troglodytes troglodytes chimpanzees infected with simian immunodeficiency virus (SIV). These chimpanzees acquired their infection from two other primates, the red-capped mangabey and the greater spot-nosed monkey. Researchers believe that the hybridisation of virus (mixing of two viruses) came about when Pan troglodytes troglodytes chimpanzees became infected with both SIV strains. This discovery was made in 1999. SIV does not appear to cause illness in chimpanzees. All HIV-1 strains are closely related to this lineage, though recent findings demonstrate that pandemic (group M) and non-pandemic (group N) HIV-1 come from distinct and geographically separate communities of the Pan troglodytes troglodytes chimpanzees.

Transmission of SIV strains between primate species seems to be a relatively common occurrence through fighting and slaughter. It is hypothesised that the virus made the leap from chimpanzee to human when there was blood-to-blood exposure while hunting or butchering chimpanzees or consuming bush meat. When an infection passes to humans from another vertebrate species, it is called a zoonotic infection. The first case of HIV-1 in humans is believed to have been around 1930.

Recent findings show that HIV-2 originated with the sooty mangabey monkey. Again, the leap from monkey to human was probably made through hunting or some other blood exposure to the virus. Researchers believe that most African primate species have been infected with a sexually transmitted form of SIV for thousands of years and that these viruses no longer harm their hosts. However, when one type of SIV crosses into a different species, it often does cause illness.

At one point, there was a hypothesis that HIV originated in Africa from contaminated polio vaccines, which may have been made with chimpanzee kidney cells and used in Africa in 1950. This theory was disproved when stored polio vaccine was tested and failed to show the presence of either HIV or SIV. Later DNA analysis of the vaccine did not find any evidence of chimpanzee tissue present. Additionally, there is fairly reliable evidence now that HIV was in the human population before the polio vaccine programme took place in Africa.