Recommendations for returning to exercise after respiratory infections, such as the flu, cold, or bronchitis, are vague. One main reason is that each person’s response to illness varies. But here is general advice.
Healthy people who have a cold without fever may continue to exercise during the illness. However, you will halve the intensity and duration. If you feel fine later in the day after your easier workout, you can gradually increase how much you do during the next session. However, if you feel exhausted after exercising, take an extra day off before exercising again.
With the flu that causes high fever, muscle aches, and fatigue, wait until the fever is over before you start exercising again. Your first training back should be light so that you do not get out of breath, and you want to move slowly when you return to your routine. You may be tempted to step up, but it is best to walk low (low intensity) and walk slowly (short duration).