Although exercise is good for the immune system, you also need to realize that if you don't give your body enough rest, it will actually hurt your immune system. Excessive exercise weakens your immune system, increases systemic inflammation and increases the risk of catching a cold. Olympic Games and other well-trained athletes often report that they are more likely to catch a cold after intense training and competition. High-intensity or long-term exercise will indeed increase the output of so-called stress hormones, alcohol and adrenaline, inhibit various elements of the immune system, raise blood pressure, raise cholesterol levels, and temporarily weaken the immune system. Some studies show that people who run 40 miles a week are more likely to catch a cold (or other upper respiratory infections) than those who run less than 10 miles. High-intensity exercise (marathon, endurance race) of 90 minutes or more makes people more likely to get sick within 72 hours after exercise. Long-term strenuous exercise increases the number of white blood cells and the existence of stress-related hormones. Marathon and triathlon athletes are particularly susceptible to infection, although susceptibility does not automatically lead to infection.