The first recognised cases of Aids occurred in America in the summer of 1981, and soon afterwards in Africa. In the beginning doctors did not know what caused this disease, and it was only in 1983 that scientists discovered that Aids is caused by a virus - now known as HIV-1.
There are two viruses associated with Aids, namely HIV-1 and HIV-2.
HIV-1 is associated with infection in most parts of Africa, America, Europe and the rest of the world, while HIV-2 occurs mostly in West-Africa. There are many subtypes of HIV-1 (for example subtype C which is most prominent in South Africa).
There are many theories about the origin of Aids, but the truth is that nobody really knows where Aids came from. The most widely accepted theory is that HIV crossed the species barrier from primates to humans at some time during the twentieth century.
HIV is related to a virus called SIV (simian immunodeficiency virus) which is found in primates such as chimpanzees, Macaque and African green monkeys. The virus probably crossed from primates to humans when contaminated animal blood entered cuts on the hands of humans who were butchering SIV-infected animals for food.
While the initial spread of HIV was probably limited to isolated communities who had little contact with the outside world, various factors such as migration, improved transportation networks, socio-economic instability, multiple sexual partners, intravenous drug use and an exchange of blood products, ultimately caused the virus to spread all over the world.