Although regular exercise has been linked to a strong immune system, people with flu symptoms, such as fever, extreme tiredness, muscle aches and swollen lymph glands, should avoid physical exertion while sick and for two weeks after they recover, according to the American Council on Exercise.

With the United States battling widespread flu activity, the council issued guidelines for athletes and others who exercise regularly. Above all else, flu sufferers should get plenty of rest, the group said in a news release.

Endurance athletes, including marathon runners and triathletes, need to be extra careful, the council added. For 72 hours after a 90-minute, high-intensity endurance workout, these athletes may be vulnerable to illness. Intense workout routines temporarily elevate levels of the hormones cortisol and adrenaline, which lowers immunity, the experts explained.

For people with symptoms of the common cold, such as runny nose, headache and mild cough, moderate exercise such as walking is considered safe. The group noted, however, that it's a good idea for people with a cold to listen to their body and reduce the intensity of their workout until their symptoms are gone.
Anyone with a cold should avoid working out at a gym or in a group environment to reduce the risk of spreading their illness to others, the council said.