banGbro said:
When you're not feeling well, it's difficult to predict whether exercising will make you feel better or worse. Most people are reluctant to stop exercising when they're sick because it can be difficult to resume their regimen. However, there are times when exercising during an illness can be potentially dangerous.
You should never exercise when you have a temperature of 99.5 [degrees] F or above. Working out when you have a fever can cause your body temperature to rise even higher, potentially leading to heatstroke, according to sports-medicine specialist Lewis G. Maharam, M.D., author of The Exercise High: How to Get It, How to Keep It (Ballantine Fawcett, $10).
A fever is an indication of your body fighting a virus. Exercise may cause a virus to invade the heart muscle or pericardium, the sac around the heart. When your body temperature is above normal, fluids are diminished by 5 to 10%, thus increasing the risk of dehydration and subsequent complications connected to the original illness. Therefore, always drink plenty of fluids to avoid dehydration. Also, do not attempt to cut back on food when you have a fever and can't exercise. The presence of a fever requires more calories, not less.
If you have a cold but no fever, exercising is fine. New research shows you can put as much effort into a workout when you have a cold as when you are healthy. At Ball State University in Muncie, Indiana, 45 students were infected with the rhinovirus that causes most colds. After developing cold symptoms, such as a runny nose, sore throat and/or cough, they ran on a treadmill for up to 15 minutes, as did 10 healthy students. Researchers found the lung capacity of the infected students was the same as that of the students who were not ill. In fact, the sick students reported that exercising didn't feel any more strenuous than usual.
http://www.findarticles.com/p/articles/mi_m0675/is_n5_v16/ai_21195455