The origin of HIV has been a subject of scientific research and debate since the virus was identified in the 1980s. HIV is a type of lentivirus, which means it attacks the immune system. In a similar way, the Simian Immunodeficiency Virus (SIV) attacks the immune systems of monkeys and apes. [1] Studies of some of the earliest known samples of HIV provide clues about when it first appeared in humans and how it evolved. Research found that HIV is related to SIV and there are many similarities between the two viruses. HIV–1 is closely related to a strain of SIV found in chimpanzees, and HIV–2 is closely related to a strain of SIV found in sooty mangabeys. [2]