Since first observed in 1981, the acquired im m une deficiency syndrom e (AIDS), has become an enorm ous medical, social, and political problem that requires m ajor com m itm ents of hum an and financial resources to resolve. As a new disease that prim arily affects young adults and that has very high m or tality, AIDS has generated intense public and medical interest. U nder pres sure to act as quickly as possible to find a cure or vaccine for AIDS, medical researchers have sometimes been overly optimistic about the implications o f the ir findings. As a consequence, the public and people with AIDS have sometimes been m isdirected toward false hopes or confused by revised in te r pretations based on new data. A lthough a cure or a vaccine has not yet been found, m uch has been learned about the epidemiology, routes o f transm is sion, natural history, and clinical manifestations o f AIDS. We now know that the life-threatening diseases associated with AIDS are end-stage m anifesta tions o f infection with the hum an im munodeficiency virus (HIV). As we learn m ore about the life cycle o f H IV , it is becom ing possible to develop and test antiviral drugs and im m une system m odulators to halt progression o f the retroviral infection and correct the im m une deficiency.