AIDS stands for acquired immunodeficiency syndrome. AIDS is the final stage of HIV infection. It can take years for a person infected with HIV, even without treatment, to reach this stage. Having AIDS means that the virus has weakened the immune system to the point at which the body has a difficult time fighting infection. When someone has one or more specific infections, certain cancers, or a very low number of T cells, he or she is considered to have AIDS.

Two types of HIV

In 1984, three years after the first reports of a disease that was to become known as AIDS, researchers discovered the main cause: the human immunodeficiency virus type 1 (HIV-1). In 1986, a second type of HIV, called HIV-2, was isolated from AIDS patients in West Africa, where it may have been present decades earlier. Studies have shown similarities between the two, but also suggested differences. Both HIV-1 and HIV-2 have the same modes of transmission and are associated with similar opportunistic infections and AIDS. In persons infected with HIV-2, immunodeficiency seems to develop more slowly and to be milder. Another difference is compared with persons infected with HIV-1, those with HIV-2 are less infectious early in the course of infection. The United States has few reported cases.

• Source: Centers for Disease Prevention and Control