HIV or Human Immunodeficiency Virus is a lentivirus (slow acting) which attacks the immune system. It is now accepted that HIV is a descendant of SIV (Simian Immunodeficiency Virus). Scientists identified a type of chimpanzee known as Pan troglodytes troglodytes, which were once common in West Africa, as the source of HIV infection in humans. Ages ago, chimps were being killed and eaten or their blood got into cuts or wounds on the hunter. This SIV adapted itself within new human host and mutated to become HIV. Every time it was transferred from a chimpanzee to a human host, it would have grown in that host's body differently, giving rise to a number of distinct early strains of HIV. The virus slowly spread throughout Africa over many years before eventually reaching other parts of the world.