The best guide for deciding whether or not to exercise when sick is the location of the symptoms. Specifically, are the symptoms located above or below the neck?
When the symptoms are above the neck—such as a stuffy or runny nose, headache, sore throat or sneezing—the illness is relatively mild and probably will not worsen with exercise. Sometimes, in fact, the symptoms may temporarily improve while exercising. For example, exercise may unclog a stuffy nose.
But when the symptoms are below the neck—such as a chest cold, hacking cough, muscle aches, fever, chills, nausea or vomiting—the illness is more severe and probably will worsen with exercise. In this case, rest is needed.
If the illness is mild and the choice is made to exercise, a level of intensity that’s below normal should be employed. Symptoms that worsen during exercise are a clear indication to stop.
A related topic that often gets overlooked is whether or not the illness can spread to others in the gym. Something like this shouldn’t be taken lightly as the health of others is now at stake.
When in doubt, hold off on exercising until healthy. And, of course, seek medical advice.
-American Fitness Magazine