Human immunodeficiency virus (HIV) is the cause of AIDS, an illness that interferes with the body's ability to fight infection and some types of cancer. The body fights infection with the help of CD4+ cells, and doctors measure their number in the blood to see how the virus is affecting the body. Higher CD4+ cells mean higher immunity, lower levels of infection, and better response to treatment. Doctors usually wait until CD4+ cell counts decrease below a certain level to begin a patient's treatment. Then patients continue treatment indefinitely. Treatment causes side effects, however, and there was reason to think that stopping treatment when CD4+ cell counts increase and restarting it when counts decrease (interrupted treatment) might help persons avoid some of those side effects. However, a recent study showed that persons receiving interrupted treatment were at greater risk for HIV-related infections and diseases. It is not known whether a person's risk for those infections returns to normal after he or she starts receiving treatment continuously. The question is important because many HIV-infected patients start, stop, and resume treatment even when they are told to take it continuously.