Physicians we are often asked to describe the typical course of AIDS: the severe immune deficiency that enables normally benign organisms to flourish destructively in patients. Our answer is that people are asking the wrong question. Now that AIDS is known to be caused by a virus-the human immunodeficiency virus, or HIV-the focus should be on the full course of the viral infection, not solely on AIDS. HIV causes a predictable, progressive derangement of immune function, and AIDS is just one, late manifestation of that process.