Sir William Osler, the famous Canadian medical doctor, once quipped, ''There's only one way to treat the common cold - with contempt.'' And for good reason. The average adult has two to three respiratory infections each year. That number jumps to six or seven for young children.

Whether or not you get sick with a cold after being exposed to a virus depends on many factors that affect your immune system. Old age, cigarette smoking, mental stress, poor nutrition and lack of sleep have all been associated with impaired immune function and increased risk of infection.

Keeping the immune system in good shape
Can regular exercise help keep your immune system in good shape? Researchers are just now supplying some answers to this new and exciting question. Fitness enthusiasts have frequently reported that they experience less sickness than their sedentary peers. For example, a survey conducted during the '80s revealed that 61 percent of 700 recreational runners reported fewer colds since they began running, while only 4 percent felt they had experienced more.

Further research has shown that during moderate exercise, several positive changes occur in the immune system. Various immune cells circulate through the body more quickly, and are better able to kill bacteria and viruses. Once the moderate exercise bout is over, the immune system returns to normal within a few hours.

In other words, every time you go for a brisk walk, your immune system receives a boost that should increase your chances of fighting off cold viruses over the long term.

Should you exercise when sick?
Fitness enthusiasts and endurance athletes alike are often uncertain of whether they should exercise or rest when sick. Although more research is needed, most sports medicine experts in this area recommend that if you have symptoms of a common cold with no fever (i.e., symptoms are above the neck), moderate exercise such as walking is probably safe.

Intensive exercise should be postponed until a few days after the symptoms have gone away. However, if there are symptoms or signs of the flu (i.e., fever, extreme tiredness, muscle aches, swollen lymph glands), then at least two weeks should probably be allowed before you resume intensive training.

Staying in shape to exercise
For athletes who are training intensely for competition, the following guidelines can help reduce their odds of getting sick.

1. 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.

2. 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.

3. Obtain adequate sleep. Major sleep disruption (e.g., three hours less than normal) has been linked to immune suppression.

4. 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.