HIV originated from non-human primates in West and Central Africa. The predominant strain, HIV-1, is closely related to a simian immunodeficiency virus (SIV) found in a subspecies of chimpanzees (Pan troglodytes troglodytes) native to this region. It is believed that HIV-1 was introduced into humans when hunters were exposed to the blood of infected chimpanzees, likely through hunting or butchering activities

. Genetic studies suggest that the cross-species transmission of SIV to humans, resulting in HIV-1, occurred around the early 20th century, approximately between 1890 and 1920, with Kinshasa (formerly Léopoldville) in the Democratic Republic of Congo being a key early epicenter of the virus's spread

. HIV-2, a less transmissible type mostly confined to West Africa, originated from SIV found in sooty mangabey monkeys, with multiple independent transmissions to humans

. The virus likely spread in human populations facilitated by social changes, urbanization, and medical practices such as the reuse of needles in vaccination programs during the early to mid-20th century. This combination of factors allowed the virus to adapt and become transmissible between humans, leading to the global HIV/AIDS pandemic

. In summary, HIV came from primate viruses (SIV) in West-Central Africa, crossing over to humans through exposure to infected blood in the early 1900s, with the virus evolving and spreading in human populations from that point onward