A chimpanzee version of the HIV virus, known as simian immunodeficiency virus (SIV), was believed to be transmitted to humans when chimpanzees were hunted for meat in West Africa, causing humans to come into contact with blood of the infected animals. After humans acquired it, the virus is believed to have mutated (changed its genetic material) to its present form. This transfer to humans may have taken place as early as the late 1800s. The virus spread across Africa and finally to other parts of the world.