You Asked: Should I Exercise When I'm Sick?

If you want to protect yourself from colds and flu, regular exercise may be the ultimate immunity-booster. Studies have shown that moderate aerobic exercise—around 30 to 45 minutes a day of activities like walking, biking or running—can more than halve your risk for respiratory infections and other common winter maladies.

There’s some evidence that very intense exercise—running a marathon, say—can briefly suppress your immune function, says Dr. Bruce Barrett, a professor of family medicine at the University of Wisconsin School of Medicine and Public Health. But in general, physical activity is a great way to shield yourself from illness, he says.

Other experts agree. “Your immune system needs activity to do its job better,” says David Nieman, a professor and director of the Human Performance Lab at Appalachian State University. “Every time you exercise, you increase the circulation of important immune cells.”

But once you’ve caught a bug and are feeling crummy, the story changes. “Exercise is great for prevention, but it can be lousy for therapy,” Nieman says.

Research from Ball State University shows that moderate exercise has no effect on the duration or severity of the common cold. “If your symptoms are neck up—things like sinus and nasal congestion, sort throat, etc.—exercise neither helps nor hurts,” Nieman says. If you feel up to it, there doesn’t seem to be much harm in continuing to work out, he adds.

But if you have the flu or other forms of fever-causing systemic infections, exercise is a bad idea.

“Back in the 1940s during the polio epidemic, some investigators noticed that athletes who played a hard game of football were coming down with the more severe form of polio,” Nieman says. This observation led to follow-up studies on the way viruses respond to exercise in both primates and humans. The sum and substance of those studies, Nieman says, is that a body infected with flu can react very poorly to physical activity. “A lot of athletes have this idea that, if I have a fever, I should sweat it out,” he says. “That’s the craziest idea ever.”