Like other viruses, retroviruses cannot replicate without taking over the biosynthetic apparatus of a cell and exploiting it for their own ends. What is unique about retroviruses is their capacity to reverse the ordinary flow of genetic information, from DNA to RNA to proteins. By 1980, the first human retrovirus, human T-lymphotropic virus type 1 (HTLV-1), was isolated; 2 years later, HTLV-2 was isolated. Both are spread by blood, by sexual intercourse, and from mother to child. Both cause disease after a long latency, and both infect T-lymphocytes. When AIDS was discovered in 1981, the likeliest agent was determined to be a retrovirus. That HIV is the cause of AIDS is now firmly established. Studies of how the virus infects both T4 cells and macrophages indicate that the CD4 molecule has a significant role in the immune function of T4 lymphocytes and also serves as a marker for that group of cells. Hence, only cells bearing that marker can be infected. In evolutionary terms, HIV-2 is clearly related to HIV-1, the virus responsible for the main AIDS epidemic. The origin of HIV is discussed, and preventive approaches are considered, including drug research and development and education. 7 figures.