Fig. 1. The natural history of HIV Infection. Clinical phases include primary infection, clinical latency, and acquired immunodeficiency syndrome (AIDS). AIDS is characterized by low CD4+ T-lymphocyte counts (less than 200 cells/mm3), high viral titers, and increased susceptibility to opportunistic infections and unusual neoplasms. (Modified from Fauci AS, Pantaleo G, Stanley S, Weissman D: Immunopathogenic mechanisms of HIV infection. Ann Intern Med 124:654, 1996)