There is some modern evidence that qi gong has immunity-boosting powers, as well: A 2011 University of Virginia study found that varsity swimmers who did qi gong at least once a week came down with 70 percent fewer respiratory infections that their teammates who practiced it less often.
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Worst: Endurance Running
Training for a marathon? Skip this weekend's long run if you're sick—even if you're already getting over, or just feel yourself coming down with, a cold. "In general, regular exercise stimulates the immune system and helps keep us healthy," says Hulse. "But too much regular exercise at a high intensity can have the opposite effect," she adds.
While no studies have looked at the effects of endurance running while already sick, Hulse says, its overall strain on the immune system is well documented: A 2007 study published in the Journal of Applied Sciences, for example, reported that immune function may be compromised for up to 24 hours after prolonged, continuous exercise (1.5 hours or longer).
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The body releases the stress hormone cortisol while it's fighting infections like the common cold, and research suggests that stress-relieving techniques—such as yoga and breathing exercises—may help boost immunity. Plus, says Besser, gentle stretching may help relieve aches and pains related to colds and sinus infections.
Choose a slower style of practice, like Hatha or Iyengar yoga, if you're worried about overdoing it with vigorous sun salutations. Or focus on restorative postures, like Child's Pose and Legs Up the Wall, at home. And don't forget to say "om": A Swedish study found that humming is a good way to open up clogged sinus passages.
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Worst: Machines at the Gym
In addition to how you exercise when you're sick, it's also important to consider where you exercise: "If your workout involves going to the gym and being in close contact with other people, you need to ask yourself if you'd want someone else with your symptoms doing the same thing," says Besser.
"If you would not like the person next to you on the treadmill or who finishes before you on the elliptical to be sneezing and coughing and wiping their nose, than do your fellow gym mates a favor and do a lighter workout at home, instead." Germs can spread easily on machines and in the locker room, he adds, so it's best to stay away while you're contagious.
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Taking a Zumba or cardio dance class—or even just rocking out to your favorite tunes while you clean the house—can serve as a stress-reduction technique. In fact, one study found that people who just listened to 50 minutes of dance music had less cortisol and more cold-fighting antibodies—a sure boost to their immune systems.