Battling another cough or cold? Feeling tired all the time? Taking a daily walk or following a simple exercise routine a few times a week may help you feel better.

Exercise not only helps your immune system fight off simple bacterial and viral infections, it decreases your chances of developing heart disease, osteoporosis, and cancer.

We don't know exactly how exercise increases your immunity to certain illnesses, but there are several theories.

• Physical activity may help by flushing bacteria out from the lungs (thus decreasing the chance of a cold, flu, or other airborne illness) and may flush out cancer-causing cells (carcinogens) by increasing output of wastes, such as urine and sweat.
• Exercise sends antibodies and white blood cells (the body's defense cells) through the body at a quicker rate. As these antibodies or white blood cells circulate more rapidly, they could detect illnesses earlier than they might normally. The increased rate of circulating blood may also trigger the release of hormones that "warn" immune cells of intruding bacteria or viruses.
• The temporary rise in body temperature may prevent bacterial growth, allowing the body to fight the infection more effectively. (This is similar to what happens when the body has a fever.)
• Exercise slows down the release of stress-related hormones. Stress increases the chance of illness.

While exercise is beneficial, be careful not to "overdo" it. People who already exercise regularly are cautioned not to develop too vigorous a workout program in the hopes of increasing the immunity benefits. Heavy, long-term exercise (such as marathon running and intense gym training) could actually decrease the amount of white blood cells circulating through the body and increase the presence of stress-related hormones.

Studies have shown that the people who benefit most from starting (and sticking to) an exercise program are those who go from a sedentary ("couch potato") lifestyle to a moderately energetic lifestyle. A moderate program can consist of:

• Bicycling with the children a few times a week
• Daily 20 - 30 minute walks
• Going to the gym every other day
• Playing golf regularly

Exercise can help us feel better about ourselves, just by making us feel more energetic and healthier. So go ahead, take that aerobics class or go for that walk -- and feel better and healthier for it.