There's an interesting relationship between exercise and illness. Generally, exercise is good for the immune system. People who don't exercise or exercise very little are sick more often, and stay sicker longer than exercisers. Moderate exercisers have fewer infections such as colds, flu and stomach bugs. However, people who exercise more than 8-10 hours a week become more prone to illness than even non-exercisers.
Moderate exercise stimulates the immune system, boosting the number and activity of infection-fighting immune cells. But, researchers aren't exactly sure why people who exercise more, experience increased illness. Some reasons include more mental stress, less sleep, increased travel, weight loss and inadequate nutrition that come with an aggressive exercise regimen.
Also, following strenuous exercise athletes enter a period of time during which they are more susceptible to illness; this is referred to as the ‘open window’ and seems to vary from 3 to 72 hours.