I'm presently battling my way through a chest cold so this question was of interest to me too so I did a bit of a Web search. The two quotes I've cut and pasted below seem to sum up the general consensus out there:
When you are sick, you need to pay careful attention to what your body is telling you. If the cold is primarily in your head, it's okay to exercise, just at a lower intensity (provided you feel okay and have enough energy). Once you are feeling 100% again, then you can slowly increase your activity back to your pre-illness level. If your feeling sluggish or the workout isn't going well, don't push it. When you push too much, the sickness can end up hanging around longer.
If your cold is below the neck (in your chest), try to refrain from activity until it clears up. The best thing you can do for your body when it's trying to fight a bug is to give it rest.
Written by Jen Mueller, Certified Personal Trainer
Most people who come down with a cold should be able to continue exercising — as long as they're up for it, says Dr. Norman Edelman, chief medical officer for the American Lung Association.
"I don't know of any evidence that exercising during a cold is bad for you," he says.
Still, a stuffy head and the Stairmaster may not mix very well. "It may be harder to breathe," Edelman explains.
That may be particularly true for people with asthma. The common cold can inflame the airways, and when people with asthma exercise they may experience chest tightening. Exercising outdoors in the winter cold can further exacerbate the problem.
With the flu, it may be hard to think about exercising when you're aching and feverish. So it's best to rest and recuperate, experts say.
"You're not going to feel like exercising with the flu," Edelman says. "You're going to feel like crawling into bed."
As for how long to wait before hitting the gym again, Edelman recommends avoiding anything too strenuous for at least a week after the flu is gone. You may have to start slowly when you begin exercising again, but a couple of weeks off isn't enough to undo all the hard work you've done to get in shape.
As a general rule, the American College of Sports Medicine says, if your symptoms are from the neck up (e.g. sniffles, sneezing), it's OK to exercise, although mild to moderate activity such as walking is best. If the illness is systemic, as with the flu, it's best to take it easy.
The bottom line, says Edelman: "Follow your body." If you're sick with the cold or flu and you don't feel like doing anything more than surfing the channels from your couch, go ahead — without the guilt.
By Jacqueline Stenson