The documented effects of physical activity on host defense date to the early 1900s, although the influences of exercise on infectious and neoplastic disease susceptibility in healthy persons have long been subject to widespread disagreement. Even less is known of the effects of exercise in reducing illness predilection for persons already susceptible to, or afflicted by, immune disease or dysfunction. Recently, however, reports suggest that exercise may reduce the incidence and severity of infection. These reports have been bolstered by well-conducted epidemiologic and laboratory studies demonstrating a link between moderate exercise and either heightened immune function or reduced disease incidence, a relationship that is reversed in instances of immoderate exercise. This symposium will present current information addressing beneficial and detrimental influences of exercise on immune function and disease susceptibility. The authors will emphasize direct effects of acute exercise and physical training on immune cells and their neuroendocrine/immune modulators. The relationship among exercise, infection susceptibility, and immune system function will be highlighted, as well as exercise-induced activation of, and illness/disease regulation by, natural killer cells. Consequences of exercise on immune function and disease advancement will be addressed for persons with neoplasia, autonomic dysfunction, aging-related immunosenescence, and AIDS.