Exercising regularly and consistently not only makes your body stronger, but it can also boost your immune system response. Moderate exercise helps raise your body temperature which can stop bacterial and viral growth. Most bacterias and viruses do not like higher body temperatures and become sluggish and their ability ability to replicate becomes stunted. By exercising you are also increasing blood circulation, which can help your white blood cells (WBC) aka fighter cells to detect infections sooner and allow your body to build up a defensive wall to block the bacteria or virus from latching on and mass reproducing itself. It is important to note, however, not to overdo it, as this can cause the opposite effect and depress the immune system.