Persons at high risk for infection by human immunodeficiency virus (HIV) include homosexuals, bisexual males, women having sex with a bisexual or homosexual partner, intravenous drug users, and hemophiliacs. The virus can be transmitted through sexual contact, use of contaminated needles or blood products, and perinatal transmission from mother to child. The antibody titer usually becomes positive 6 to 12 weeks after exposure, and the presence of the antibody provides no protection against acquired immunodeficiency syndrome (AIDS). Because of occasional delayed appearance of the antibody after initial exposure, it is important to follow up patients for 1 year after exposure.