Different levels of exercise can significantly increase or decrease your chances of catching a cold. If you tend to be a couch potato, then you will catch 2-3 colds per year, on average. Regular moderate exercise can reduce the risk of catching cold-like infections by 33% (this effect has been shown to be the result of the cumulative effect of exercise leading to long-term improvement in immunity). Prolonged strenuous exercise, such as marathons, can make an individual more susceptible to catching colds. We are constantly exposed to viruses, but it is our immune system that determines whether we get sick or not.
Association for Science Education Conference, Jan 2012