Exercising while sick is not always ill-advised. It might not be easy but going for a run when your symptoms are situated above the neck and you're fever-free may actually make you feel a little better. However, if you have a fever, you need to cool your jets. "If you're febrile, if you're over 100 degrees – definitely over 100.5 or so – you should not work out. You put yourself at some risk for viral myocarditis," says Kaeding. Viral myocarditis is inflammation of the heart muscle and can be fatal. Evidence suggests that exercising while feverish increases a person's risk of developing this dangerous disorder as well as dehydration and overheating.