In the United States, AIDS is found primarily among homosexual and bisexual males and intravenous drug users. About 3 percent of cases have been traced to transfusion of contaminated blood products, and only 1 percent of cases have been traced to heterosexual contacts. This is in contrast to the situation in Africa, where heterosexual transmission is the rule. AIDS is an infection of helper T lymphocytes by the human immunodeficiency virus. The penetration and infection of these lymphocytes by the virus results in their death and depletion with grave consequences to the immune system. A major consequence is high susceptibility to opportunistic infections and malignant diseases such as Kaposi's sarcoma. Since 1982, researchers have sought to develop vaccines and treatments for AIDS, but no agents have yet been found to be successful. Enough is now known about AIDS to suggest an alternative approach to prevention. AIDS is transmitted through intimate sexual contact and the exchange of genital secretions and can be transmitted through contaminated blood. On the basis of this knowledge, the further spread of this disease can be prevented.