For the most part, exercise helps boost and fortify the immune system, but too much of a good thing can be detrimental. Too much exercise puts strain on the body and immune system, which means you’re more susceptible to sickness. On top of that, if you’re sick for a while and have a tough time getting better, overtraining may be responsible for that too. Take some time off to let your body get rid of the illness and get the immune system back to where it should be.