The immune system is pretty complicated, but there are some things that are pretty easy to understand. One is that mounting an immune response costs your body energy and raw materials like protein.

Let’s start off by looking at how exercise in general influences the immune response. Exercise can reduce stress. This is good because stress decreases a person’s immune response. When the body is using its resources to mount a stress response, it directs energy to get ready for “fight or flight,” not germ fighting. People who exercise regularly get fewer colds than “couch potatoes.” Good nutrition, sleep, and keeping a healthy weight also seem to support a good immune response.

But what if the exercise is excessive? Athletes and researchers call this “overtraining.” After a hard workout, the body needs to replenish water. It may also need to replace carbohydrates and electrolytes (but probably not nearly as much or as often as sports drink companies may imply). The body also needs sleep because that is when it repairs the tissues that are broken down by hard workouts. It needs some protein to repair the tissue as well. If a person overtrains, the energy and raw materials get put toward repair, and there may not be enough left for an effective immune response. Pushing yourself a little harder is how you improve, but if you push yourself too much harder, for too long, you may do worse.

Athletes who overtrain start to feel tired all of the time, but may not be able to sleep very well. They do not perform well in practice and competition. They often have negative moods. Their heart rate is higher than normal when they are not exercising. They may lose their appetite, even when they are losing weight. Their immune systems are less active and they are more likely to get colds.

If you are an athlete, you can monitor yourself. Try keeping a log of your workouts, how you feel, and how you are performing. A runner may keep track of split times, for example. Your resting heart rate will also be a good measurement. When you wake up in the morning, before you get up, measure your pulse for 10 seconds, they multiply that by 6. The easiest place to feel your pulse is the side of your throat, just below your jaw. Write this down in your log too. Only a doctor can diagnose whether you have overtraining syndrome, but this information will be very useful in the diagnosis.

There is no quick fix for overtraining. It can take weeks or months to recover, depending on how overtrained the athlete is and how long they have been that way.

Don’t forget to wash your hands. You’ll create less work for your immune system.

During intense exercise, your body produces higher levels of cortisol and adrenaline, which can suppress immune function. This is a temporary effect, probably lasting a couple of days. However, if you exercise intensely all the time, the effects can be a little more harmful, since your body won't have time to recover from this lowered immune function state.

Excessive exercise can exhaust you and cause your entire body to perform less effectively, having burnt all of your reserves of energy. This affects your immunity as well as everything else.

According to some research, moderate and regular exercise may be a very good thing for your immune system. However, there is also evidence that too much intense exercise can reduce immunity. This research is showing that more than 90 minutes of high-intensity endurance exercise can make athletes susceptible to illness for up to 72 hours after the exercise session. This is important information for those who compete in longer events such as marathons or triathlons.

Intense exercise seems to cause a temporary decrease in immune system function. Research has found that during intense physical exertion, the body produces certain hormones that temporarily lower immunity. Cortisol and adrenaline, known as the stress hormones, raise blood pressure and cholesterol levels and suppress the immune system. This effect has been linked to the increased susceptibility to infection in endurance athletes after extreme exercise (such as marathon running or Ironman-distance triathlon training).