These data suggest that immune function in several body compartments exhibits sins of stress or suppression for a short period followings prolonged endurance exercise. Thus, it makes sense that URTI risk may be increased when an athlete goes through repeated cycles of unusually heavy exertion, has been exposed to novel pathogens, and has experienced other stressors to the immune system including lack of sleep, severe mental stress, URTI risk is related to many factors, and when these are brought together, the athlete may be unusually susceptible.