Physical activity influences immune function and risk of certain types of infection such as upper respiratory tract infections (URTI). In contrast to moderate physical activity, prolonged and intensive exertion lasting 90 minutes or longer by endurance athletes causes numerous changes in immunity in multiple body compartments and an increased risk of URTI. Each acute bout of heavy exertion leads to physiological stress and clinically significant changes in immunity and the capacity to detect and kill invading bacteria and viruses. Stressful levels of exercise affect both the innate and the adaptive immune system.