Here's an an article from the New York Times describing research on the effects of exercise on the immune system. The main study involved exposing different groups of mice to the flu virus. One group had no exercise, one group moderate, and one group pushed to exercise intensely. The moderate exercise group did well, and the intense exercise group did very poorly. The results tell us about mice, but we can guess that it works the same way in people -- and certainly, I recommend only moderate exercise for my patients who are simply interested in maintaining health.
When you are an athlete in competitive training, you need to take these effects into account -- especially in focusing on having adequate recovery time and adequate nutrition to support you body's training. And, also, to note that the studies describe the negative effects of 'intense exercise' on immunity. That definition is very flexible and will depend on the person doing the exercise. If you have trained well, your capacity for healthy exercise will be greater.
The key finding in this study:
The bulk of the new research, including the mouse studies mentioned, reinforce a theory that physiologists advanced some years ago, about what they call “a J-shaped curve” involving exercise and immunity. In this model, the risk both of catching a cold or the flu and of having a particularly severe form of the infection “drop if you exercise moderately,” says Mary P. Miles, PhD, an associate professor of exercise sciences at Montana State University and the author of an editorial about exercise and immunity published in the most recent edition of the journal Exercise and Sport Sciences Review. But the risk both of catching an illness and of becoming especially sick when you do “jump right back up” if you exercise intensely or for a prolonged period of time, surpassing the risks among the sedentary. (Although definitions of intense exercise vary among researchers, most define it as a workout or race of an hour or more during which your heart rate and respiration soar and you feel as if you are working hard.)
Chinese medicine has a number of herbs that help with stamina and immunity -- like Ginseng and Reishi Mushroom. Supporting the immune system before and especially after competitive events is a great idea.