Interest in infectious disease among athletes has been greatly stimulated over the past decade by the development of modern automated systems that can enumerate specific elements of the immune system. Research has confirmed earlier clinical and animal studies in showing that either a single bout of exhausting exercise or persistent over-training can increase susceptibility to upper respiratory and other viral infections, although resistance to bacterial infections is apparently unaltered. Such findings do not seem a non-specific response to cooling and drying of the tracheal mucosa. Rather, heavy exercise has a depressant effect upon the T cell/interleukin/NK cell system which may persist for a week or more. In contrast, moderate training enhances immune defences. Given the negative impact of acute viral infections upon both competitive performance and morale, plus the occasional incident of sudden death associated with viral myocarditis, it is important that sports physicians minimize the incidence of viral infections in the athletes for whom they are responsible. Potential tactics include maintenance of immunization schedules, minimizing of exposure to infection, avoidance of over-training, maintenance of an adequate diet, and reduction of psychological and environmental stress. In top athletes, the regular monitoring of immune status may also be warranted, with the possible administration of immunoglobulins and prostaglandin inhibitors as required.