Origin of AIDS

AIDS is caused by the human immunodeficiency virus (HIV), which originated in non-human primates in Sub-Saharan Africa. While various sub-groups of the virus acquired human infectivity at different times, the global pandemic had its origins in the emergence of one specific strain – HIV-1 subgroup M – in Leopoldville in the Belgian Congo (now Kinshasa in the Democratic Republic of the Congo) in the 1920s, as discovered on October 2, 2014.

Two types of HIV exist: HIV-1 and HIV-2. HIV-1 is more virulent, is more easily transmitted and is the cause of the vast majority of HIV infections globally. The pandemic strain of HIV-1 is closely related to a virus found in the chimpanzees of the subspecies Pan troglodytes troglodytes, which lives in the forests of the Central African nations of Cameroon, Equatorial Guinea, Gabon, Republic of Congo (or Congo-Brazzaville), and Central African Republic.  HIV-2 is less transmittable and is largely confined to West Africa, along with its closest relative, a virus of the sooty mangabey (Cercocebus atys atys), an Old World monkey inhabiting southern Senbegal, Guinea-Bissau, Guinea, Sierra Leone, Liberia and western Ivory Coast.

Bushmeat Practice

According to the natural transfer theory (also called 'Hunter Theory' or 'Bushmeat Theory'), the "simplest and most plausible explanation for the cross-species transmission" of SIV or HIV (post mutation), the virus was transmitted from an ape or monkey to a human when a hunter or bushmeat vendor/handler was bitten or cut while hunting or butchering the animal. The resulting exposure to blood or other bodily fluids of the animal can result in SIV infection. A study published in 2009 also discussed that bushmeat in other parts of the world, such as Argentina, may be a possible location for where the disease originated. A recent serological survey showed that human infections by SIV are not rare in Central Africa: the percentage of people showing seroreactivity to antigens — evidence of current or past SIV infection — was 2.3% among the general population of Caneroon, 7.8% in villages where bushmeat is hunted or used, and 17.1% in the most exposed people of these villages. How the SIV virus would have transformed into HIV after infection of the hunter or bushmeat handler from the ape/monkey is still a matter of debate, although natural selection would favor any viruses capable of adjusting so that they could infect and reproduce in the T cells of a human host.

http://en.wikipedia.org/wiki/History_of_HIV/AIDS

[This Wikipedia article also discusses many other alternative theories of cross-species transmission in the article linked immediately above.]