The worldwide pandemic of acquired immunodeficiency syndrome (AIDS) has the potential to cause catastrophic medical and social effects that will influence world health well into the 21st century. The causative agent, a lentiretrovirus called human immunodeficiency virus (HIV-1), is spread by intimate exposure to blood and bodily fluids through sexual, parenteral, and mother-to-infant exposure. The natural history from exposure to disease has a median incubation period of 8-10 years and is characterized by progressive depletion of CD-4 positive T lymphocytes as well as effects on other immune and central nervous system cell populations. The World Health Organization (WHO) estimates that between 8 and 10 million persons are currently infected with the virus worldwide, with 8 to 10 times this level projected by some estimates into the 21st century. Recent leveling off of AIDS incidence in the U.S. appears to represent the positive benefits of antiretroviral therapy, and considerable benefit could be seen if such therapies were made more widely available to medically underserved populations. With prolonged survival, however, other long-term sequelae such as cancer and lymphoma may emerge as significant complications of prolonged immunodeficiency. Furthermore, the large pool of already infected persons and continued spread of the virus make the development of additional therapies and an effective anti-HIV vaccine priorities of medical research.