This year, if a cold or the flu tries to interrupt your quality time at the gym, keep this in mind: you are better off taking a break instead of breaking a sweat.

Forget about what you might have heard about trying to perspire your way back to better health. That's not the way your immune system fights off sickness, says Liz Applegate, PhD, a fitness and nutrition expert on the faculty of the Nutrition Department at the University of California, Davis.

Your body can't "sweat out" toxins and germs during exercise. That's a job for the immune system, she says, which uses its complex network of specialized cells and components to engulf, detoxify, and disassemble bad guys. She says there's more you should know.

Your immune system fights most effectively when it isn't stressed. Research studies show that a moderate fitness program helps boost the immune system, lessening the chances you'll fall ill with a cold or flu. But scientists also note that a single rigorous exercise session or race can actually make you more susceptible to bacterial or viral infection.

So listen to your body when you feel sick and need to rest -- a hard workout could impair your immune system for several hours, allowing unwelcome guests to make your illness worse. And make sure you give your body enough time to recover before you return to exercise after a serious illness like the flu. Come back too soon and you may actually send yourself into a relapse of the illness, which further slows your return to everyday activities.

Here's some advice from Applegate on ways you can balance exercise and illness this winter.

• Fever. If you're hot, skip the workout. Elevated body temperature raises your heart rate, and exercise jacks up your heat and heartbeats even more, putting stress on your immune system.

• Aches and pains. Forget the exercise if you have nagging muscle pains or a headache, which could be a sign your body is trying to fight off a bug.

• Upset stomach. If you are having trouble keeping down solid food, you should not be keeping up with the exercise. Your body could use the rest, and exertion could make you feel worse.

• Hacking cough or lung congestion. It's not a pretty thought, but you could be pretty sick if you are coughing stuff up. This is a sign your respiratory system is under siege and probably would not be up to the rapid-fire breathing you usually work up to when you are in your best form.

When your illness seems to be letting up, work back into your exercise routine slowly. Before you even head back to the gym, allow three to four days of rest after a bad cold and at least a week or so after the flu.

When you do go back, plan on exercising half your usual time, going half-speed on the treadmill or stationary bike, and backing off on the weights until you regain your endurance and strength. You may need to stick with this reduced regimen a week or more, depending upon how long you were out. Judge your comeback by your fatigue level, both during your workouts and throughout the day. When you're no longer feeling over-tired, you can start working out at your pre-illness intensity.

Don't worry about losing all you've achieved from the workout room. Just because you take-five to fight off a brief illness doesn't necessarily mean you'll have to start over at square one when you are feeling well again. But if you work out too soon or too hard before you fully recover, you might find yourself back in bed again.

Then, once you're back, try to stay healthy -- these tips may help you avoid another close encounter with the sickness bugs.

• Eat your fruits and vegetables. Several servings every day will give your body the vitamin C and other nutrients that are crucial for a healthy immune system.

• Get plenty of rest. Sleep not only helps you rebuild, but also keeps your immune system functioning at its best. Aim for eight hours nightly.

• Take a few precautions. Wash your hands frequently, especially if you're in contact with many people (shaking hands, touching door knobs). Bring your own water to the gym to avoid touching the water fountain (the water's OK but the fountain itself has many users!). And bring along two towels, one to wipe the equipment, the other for your face and hands.

Source: webmd.com, 2003