Experts like to cite a rule of thumb known as the "neck rule." If your symptoms are all located above your neck (stuffy nose, scratchy throat, headache), you almost certainly have a head cold and can hit the road or treadmill safely. If, on the other hand, you have a fever, congestion in your chest and lungs, or feel achy, it is probably a sign of flu, bronchitis, or another more serious ailment, and you should rest up. (Exercising with a fever will make you more vulnerable to dehydration, among other ill effects.)