HIV infects and kills T4 cells that are depleted in AIDS by interacting with the molecule called CD4. HIV infects cells by binding to CD4; 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 molecular biology of HIV is complex; HIV is able to slip into cells and remain there for life. Its elaborate genetic regulation enables it to remain hidden from immune surveillance and to replicate slowly. Even when HIV is active, the design of its envelope protein and the variability that results from its error-prone replication mechanisms make it a difficult target for an immune response. Studies of the origin of the AIDS virus indicate that African societies may have played the most important role. In the United States, AIDS is a major cause of morbidity and mortality, and it is the leading cause of death among people with hemophilia and users of illegal intravenous drugs. By 1992, it is estimated there will be 365,000 cases of AIDS in the United States. Reports to the World Health Organization suggest that at least 5 million people worldwide are infected by the AIDS virus and that a million new cases of AIDS are likely within the next 5 years. Knowledge of the interaction between HIV and the CD4 molecule may help in developing effective AIDS therapies and vaccines. Although the AZT drug is already in clinical use, expanding knowledge of HIV will make it possible to design other drugs that interrupt specific phases of the viral life cycle. How the AIDS epidemic is dealt with will have a profound effect on society and the protection of AIDS patients from social discrimination. 42 references, 70 figures.