The first cases of AIDS were identified in the United States in 1981, but AIDS most likely existed for many years before that. Scientists have identified a type of chimpanzee in West Africa as the likely source of HIV infection in humans.
They believe that the chimpanzee version of the virus was most likely transmitted to humans and mutated into HIV when humans hunted these chimpanzees for meat and came into contact with their infected blood. Over the decades, the virus has slowly spread across Africa and to other parts of the world.