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AIDS or acquired immunodeficiency syndrome, fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981. The virus was isolated in 1983 and was ultimately named the human immunodeficiency virus (HIV). There are two forms of the HIV virus, HIV-1 and HIV-2. The majority of cases worldwide are caused by HIV-1. In 1999 an international team of genetic scientists reported that HIV-1 can be traced to a closely related strain of virus, called simian immunodeficiency virus (SIV), that infects a subspecies of chimpanzee (Pan troglodytes troglodytes) in W central Africa. Chimpanzees are hunted for meat in this region, and it is believed the virus may have passed from the blood of chimpanzees into humans through superficial wounds, probably in the early 1930s.
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