If you're the kind of exerciser who is hooked on your daily "fix" of activity and you'd rather miss the Second Coming than your aerobics class, you probably wouldn't let a few sneezes or a scratchy throat keep you on the sidelines.

But before you rush out to "just do it" when you're sick, says E. Randy Eichner, a marathon runner and professor of medicine at the University of Oklahoma Health Sciences Center, "ask yourself, 'What's the point?' " If you're exercising for your health and because it makes you feel good, he says, "why work out when your body's telling you to rest?"

Eichner doesn't work out when he has a cold, he says, but he usually permits athletes to train when they have colds as long as they do not have a fever. For many competitive athletes, the stress of missing a workout can be worse than the stress of exercising when they feel under the weather. But Eichner sidelines athletes suffering from more severe cold symptoms or the flu, because, he notes, "some viruses may invade your heart."

For those who can't decide whether to work out or not, he offers these guidelines:

If the symptoms are located above the neck, such as runny nose, sneezing, scratchy throat, then exercise is usually safe. "I recommend taking a 'test drive,' by going for about 10 minutes or so at about half speed," Eichner says. "If your head seems to clear and you feel peppy, then it's okay to finish. But if your head keeps pounding and you feel awful, I don't see the point in continuing."

Never exercise with below-the-neck symptoms such as fever, muscle aches, loss of appetite or a hacking cough that produces phlegm in your throat.

"Laboratory experiments have shown that animals infected with a flu virus can develop a serious heart inflammation when they are forced to swim," says Harvey Simon, a physician in the infectious disease unit at Massachusetts General Hospital in Boston. "If you're running a fever, avoid even moderately strenuous exertion until your temperature is back to normal. After that, you can start moving again, but give yourself a few days to build up to where you were before."

While the potential for heart problems is the most serious risk of exercising with a fever, dehydration is also a possibility. "With a fever, you tend to lose fluid," notes Todd Miller, a cardiologist at the Mayo Clinic in Rochester, Minn. Combined with the fluid loss from exercise, he says, "it can be very taxing on the system."

In addition to checking your temperature, Miller advises, check your resting pulse rate if you're feeling below par. If it's higher than usual, he says, take a day off.

Unlike fevers, head colds can sometimes be helped by exercise, some fitness enthusiasts claim. "It's like eating Chinese food," Miller says. "You can clear out your head some with a run." But be cautious about taking cold medications before exercising, he warns; one side effect of some cold pills is an elevated heart rate. Also, antihistamines will dry up mucous membrances that you need to stay moist. And they can have a sedating effect.

"If you feel bad enough to pop cold pills," Miller says, "you may be better off not exercising that day."

Strength-trainers need to be wary of intense activity with a cold, since "you can damage your muscles by weight training when you're sick," says New York exercise physiologist and gym owner John Comereski. "Your body is already working to fight the illness, and it is added stress to do a heavy workout. The problem is, many regular exercisers don't know how to cut back or take a short layoff when their bodies are fighting a cold."

Habits are hard to change, even good ones like the exercise habit. But it's important to build flexibility into your workout routine to avoid injury. If you're not too sick, try substituting a lower-intensity activity, such as moderate walking. But if you've got the below-the-neck symptoms that cry out for rest, just remember that the break is temporary and rest can be rejuvenating.

"If you don't listen to your body and take a few days off or slow down when you need to," says Comereski, "you could be looking at a serious injury. Then exercising or not won't be a choice."

Bodyworks appears on alternate Tuesdays.