If you're still in doubt about whether it's safe to run or not, take your temperature. If it's above 99 degrees, skip your run. "Some people think that they can 'sweat out' a fever by running," says Nieman. "That's wrong. Running won't help your immune system fight the fever."
Nieman saw this firsthand when his running partner once ran a marathon with a 101-degree fever. Soon after, the runner developed severe and persistent symptoms similar to those of chronic fatigue syndrome. "Every day he'd wake up feeling creaky and arthritic," says Nieman. "When he tried to run, he'd stumble and fall." Eventually doctors concluded he had a "postviral syndrome," a latent condition that was exacerbated by the race.
Although this syndrome is rare, it's an example of the risk you take by running while ill. "Running with a fever makes the fever and flu-like symptoms worse," says Nieman, "and it can lead to other complications." During exercise, your heart pumps a large amount of blood from your muscles to your skin, dissipating the heat your body generates. If you have a fever, your temperature will rise even higher, and your heart will be put under greater strain to keep your temperature from soaring. In some cases, this can produce an irregular heartbeat. Also, a virus can cause your muscles to feel sore and achy; exercising when your muscles are already compromised could lead to injury.
Nieman recommends that runners with a fever or the flu hold off until the day after the symptoms disappear–and then go for a short, easy run. Runners should wait one to two weeks before resuming their pre-illness intensity and mileage. Otherwise, you risk a relapse, he says.
Above all, obey your body and the thermometer–not your training program.
Know Your Limits
How much running can compromise your immune system to the point of making you sick? For average runners, the dividing line seems to be 60 miles a week, according to David Nieman, Ph.D., of the Human Performance Laboratory at Appalachian State University. Nieman conducted the largest study ever done on this question by examining 2,300 runners who competed in the 1987 Los Angeles Marathon. "The odds of getting sick were six times higher than normal after the marathon," says Nieman, "and those who ran 60 miles a week or more doubled their chance of getting sick." The illnesses were of the upper respiratory tract, including sinus infections. Nieman says there's no doubt these findings are still applicable to runners today. He's also used himself as a test case: When Nieman trained up to 90 miles a week, he constantly battled sore throats. When he dropped his weekly mileage below 60, the symptoms stopped.