Be active, stay healthy.
Good physical conditioning and lifestyle that includes enough exercise strengthen the immune system. This association may however have a U-shaped response curve;
intensive exercise reduces the chance of catching a cold, but very intensive exercise increases the chance for period after following such exertion. In athletes, a common observation is that symptoms of respiratory illness take place around prolonged exercise, periods of intensified training or competitions.
This study tracked the relationship between upper respiratory track infections, their severity and frequency of aerobic activity. The researchers concluded that 'perceived physical fitness and frequency of aerobic exercise are important correlates of reduced days with upper respiratory track infections and severity of symptoms’.