Eat a well-balanced diet. The immune system depends on many vitamins and minerals for optimal function. However, at this time, there is no good data to support supplementation beyond 100 percent of the Recommended Dietary Allowances. A good rule is to eat 10-15 calories per pound of "desired body weight." If your ideal weight is 170 lbs, then consume 1700-2550 calories a day (1700 for sedentary individuals and 2550 for extremely active types.)
Avoid rapid weight loss. Low-calorie diets, long-term fasting and rapid weight loss have been shown to impair immune function. Losing weight while training heavily is not good for the immune system.
Obtain adequate sleep. Major sleep disruption (e.g., three hours less than normal) has been linked to immune suppression.
Avoid over-training and chronic fatigue. Space vigorous workouts and race events as far apart as possible. Keep "within yourself" and don’t push beyond your ability to recover.
Wash your hands frequently. This is often your best prevention. Don't forget your fingernails.
Avoid putting your hands near your eyes, nose or mouth. Most bacteria and germs are spread from a surface to your hands to your face not by air.
Get a flu shot. Especially if you have a weakened immune system.
Get enough sleep. Lack of sleep can reduce immune functioning making you susceptible to sickness.
Drink more water. In the fall and winter, it is easy to overlook your thirst and get dehydrated. Make sure you consume 8 glasses a day.
Continue a moderate exercise program. Try to maintain a consistent exercise routine
Limit alcohol intake. Alcohol can be dehydrating which, in turn, may decrease your resistance to bacteria.
Finally, listen to your body. If you are less than 100% you will feel better and recover faster if you let yourself rest. 