For people who have a cold or simple upper respiratory infection without fever or significant cough, exercise is generally safe. You want to cut your intensity and duration of exercise in half. If you feel good over the next few hours, you can increase how much you do the next time you work out. However, if you feel exhausted after exercising, then skip two to three days before exercising again.
With flu or other illness that causes high fever, muscle aches and fatigue, wait until the fever is gone before getting back to exercise.
The first workout after this kind of illness should be very light, making sure that you are not getting short of breath. You will want to progress more slowly as you return toward your normal routine. If you usually exercise daily, I advise working out every other day for the seven to 10 days after illness.
Viral infections, such as the flu, can cause temporary muscle weakness. This includes the muscle cells in the heart. Exercising won’t cause a heart infection. But a heart weakened by a viral infection could be hurt by strenuous exercise. Go low (low intensity) and go slow (short duration). If you feel any shortness of breath, this is not the time to push it.