This paper examines whether physical activity influences immune function as a consequence risk of infection from the common cold and other upper respiratory tract infections (URTI) and whether the immune system responds differently to moderate versus intense physical exertion. Research indicates that people who participate in regular moderate exercise report fewer colds than their sedentary peers. Fitness enthusiasts are often uncertain of whether they should exercise or rest during sickness. Human studies are lacking to provide definitive answers, but animal studies generally support the finding that one or two periods of exhaustive exercise following exposure to viruses or bacteria lead to more frequent appearances of infection and more severe symptoms. With athletes, it is well established that sickness reduces the ability to compete. A common perception among endurance athletes and coaches is that overtraining lowers resistance to URTI. Immune function changes dramatically after each bout of prolonged, intensive exercise. Attempts to compare resting immune function in athletes and nonathletes have failed to provide evidence that athletic endeavor is linked to clinically important changes in immunity, despite compelling epidemiological data. Based on current knowledge, good immune function can be maintained by regular physical activity, eating a balanced diet, minimizing stress, avoiding chronic fatigue, and obtaining adequate sleep. (Contains 35 references.) (SM)