The HIV epidemic appeared rapidly, and the United States is now its epicenter. Its incidence and main transmission routes vary considerably among different countries. HIV is not spread by casual contact, but its transmission mainly through drug use and sexual intercourse pose major challenges for efforts at prevention through education and behavior modification. In the United States, homosexual men are at the greatest risk of HIV infection, although their self-generated educational programs have dramatically reduced the rate of spread of the virus. Intravenous drug users who share needles are the next largest risk category; prevention programs such as needle exchange remain controversial despite evidence supporting their effectiveness. In addition, serologic screening for HIV antibodies in several populations suggest that heterosexual spread of HIV may become increasingly important in the future. Thus, the epidemic will have a massive impact for the next 5-10 years, and effective antidiscrimination policies must be recognized as an essential component of effort to address it.