Recent research studies and other evidence suggest that although moderate exercise is good for the immune system, the demanding training programs of many top athletes may suppress the immune system and thereby increase susceptibility to infections. A number of top athletes have suffered from unusual infections normally associated with immune deficiency, and immune abnormalities have been demonstrated in resting samples from top athletes. Studies from several exercise laboratories have shown that after a single exhausting exercise session there is temporary immune depression, with marked changes in numbers and functional capacities of lymphocytes. These changes, which last for up to several hours, are seen in athletes and untrained individuals. In several studies in the United States, students who were very active in sports have been shown to be more susceptible to infections than their less active colleagues. Exercising hard during the incubation phase of an infection can increase the severity of the illness. This article examines the evidence, discusses possible mechanisms, and considers the implications.