Human immunodeficiency virus (HIV) is a retrovirus that can be transmitted through sexual activity, blood products, and perinatal exposure. The virus is composed of core, transmembrane, and envelope proteins. Cells of the immune system are the primary target of HIV, and destruction of the immune response is characteristic of end-stage disease. Although male homosexuals continue to represent the largest population of persons with acquired immunodeficiency syndrome (AIDS), transmission among intravenous drug users accounts for the rapidly growing incidence of pediatric and heterosexual AIDS patients. Control of the epidemic among intravenous drug users is the major challenge in the US today.