Yes, moderate exercise can improve your immune system. People who exercise tend to have a stronger immune system and have lower incidence of heart disease, diabetes, some forms of cancer, and even fewer colds. Low -intensity exercise (e.g. walking) has been shown to help fight off simple forms of bacterial and viral infections. It is theorized that exercise boosts white blood cells. White blood cells are the body's defense against disease and infection. But be careful not to overdo it. Vigorous exercise for extended periods of time (especially if you're new to exercise) can cause upper respiratory tract infections and muscle soreness. Start slow and gradually progress your exercise routine. You'll feel better in no time.