While moderate exercise is known to be very beneficial, exceptionally strenuous exercise presents special challenges.

"There is still a lot to learn about how exercise affects the immune system, because it's difficult for researchers to assess the many layers of protection within the system," Flynn says. "Strenuous or prolonged exercise seems to suppress the immune system, leaving athletes more susceptible to illness for one to six hours following a hard workout - the so-called open window."

Serious athletes, such as those who run 40 miles a week, have a higher rate of upper-respiratory tract infection than recreational joggers, Flynn says.

For serious athletes to get the most from their training routine, and to be able to fight off illness, it is essential for them to take some time off between hard workouts, he says.

"Rest days are as important as training days, and it's important for endurance athletes to speak with their personal physicians about their routine," Flynn says. "Athletes also should be aware of this post-exercise window of opportunity, continue to eat right and take precautions around others who are sick."