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Medical Update : If You're Concerned About Exercising During a Cold . . . : Edwin W. Brown, M.D., M.P.H.
Healthy immune systems can cope with mild infections without being compromised by moderate workouts. A recent study found that such workouts neither helped nor hindered the length or severity of colds--although some researchers think that mild exercise may stimulate the immune system. Vigorous workouts, however, may worsen the situation, as demonstrated in animal studies. If you have a cold and still want to work out , begin at about half your normal intensity , then pick up the pace slightly if you still feel OK. In any case , don't go through your full normal routine. However , don't try it with such more serious infections as pneumonia , bronchitis , influenza , etc. , even if you don't feel too badly--which isn't likely! Any amount of exercise puts a strain on the immune system in such illnesses. Even after symptoms disappear , it's best to keep physical activity down to moderate levels for at least two weeks.