A pattern of highly unusual infections in otherwise healthy young adults emerged in the early 1980s. This pattern, or syndrome, was caused by an unknown entity that apparently attacked the body's immune system. It became known as AIDS. Between 1983 and 1984, researchers isolated a new virus - HIV - the cause of AIDS. This made possible a blood test for antibodies to the virus. HIV was found to be an infectious agent known as retrovirus. Different retroviruses were found in some animals but, until that point, were rare in humans. HIV may have been infecting some human populations relatively benignly for more than 20 years.
     Since the discovery of HIV, several strains of the virus have been identified. In 1985, a related virus was found in parts of West Africa and was called HIV-2 to distinguish it from the earlier virus HIV-1. The pattern of illness is similar for both HIV-1 and HIV-2.
     In the early 1980s, only about 100,000 adults worldwide were thought to have been infected with HIV. As of the end of 1998, the number of adults and children living with HIV or AIDS rose to more than 33.4 million.