Q: When you've have a cold or the flu, how do you know when it's safe to continue a routine exercise program?

A: Those who work hard to keep up a regular exercise program -- during or just after illness -- often ask this question.

It's important to think about the intensity of your exercise program and your general health before the infection. For instance:

Someone in excellent health may be able to get back to exercise sooner than someone who has multiple medical problems.

Seniors may take longer to recover from a viral infection (such as a cold or flu) than younger people.

The more intense the exercise routine, the longer it takes to come back to that level of exercise after an infection. If you are training for a marathon, a longer rest period followed by a less-intense exercise may be appropriate. On the other hand, if your exercise program mostly involves stretching, you may be able to exercise again right away.

The specific type and severity of the infection play an important role in determining when or if to exercise. If the infection was mild and brief, re-starting exercise within a few days is probably safe.

But if there were more significant symptoms -- such as high fever, severe fatigue, cough, wheezing or other respiratory symptoms -- a longer recovery period (perhaps 1 to 2 weeks, or even more) might be best. Clearly there are times when taking a break from exercise makes sense For example, if you have a fever, most doctors would recommend rest.

There are no hard and fast rules to tell for sure if you're ready to get back to your regular exercise program. The best plan is to let your symptoms be your guide. When you're feeling back to normal or close to it, returning to your exercise program at a slightly lower intensity is safe in most cases.

The good news is that a viral infection is almost always a minor setback. And you won't lose the benefits you've gained from routine exercise due to this brief period of rest.

(Robert H. Shmerling, M.D. is a practicing physician in rheumatology at Beth Israel Deaconess Medical Center, Boston, Mass., and an Associate Professor in Medicine at Harvard Medical School.)

(For additional consumer health information, please visit http://www.health.harvard.edu.)