Scientists identified a type of chimpanzee in Central Africa as the source of HIV infection in humans. They believe that the chimpanzee version of the immunodeficiency virus most likely was transmitted to humans and changed into HIV when humans hunted these chimpanzees for meat and came into contact with their infected blood. Studies show that HIV may have jumped from apes to humans as far back as the late 1800's. Over the decades, the virus has slowly spread across Africa and later into other parts of the world. We know the virus has existed in the US since at least the mid to late 1970's.