Proponents of exercise often tout its supposed immune system-boosting effects. However, in the short term, intense exercise has been shown to increase risk of respiratory infections.
Its not uncommon (especially from power lifters) to feel feverish the morning after a particularly intense workout session or for competitive runners to reach for Tylenol. A classic study from the Journal of Athletic Training notes that:
The epidemiologic data suggest that endurance athletes are at increased risk for URTI during periods of heavy training and the 1-to 2-week period after marathon-type race events. Several researchers have reported a diminished neutrophil function in athletes during periods of intense and heavy training. Following each bout of prolonged heavy endurance exercise, several components of the immune system appear to demonstrate suppressed function for several hours.
In other words, during periods of heavy endurance training, the immune system is suppressed resulting in a 3-12 hour “window” of elevated vulnerability.
Dr. David C. Nieman, Professor at Department of Health, Leisure, and Exercise Science at Appalachian State University recommends athletes practice careful hygiene to lower risk for infection and avoid training during times of illness.