According to most experts, working out with the flu does more harm than good.
No Such Thing as Sweating it Out
“The best information we have says complete bed rest is best,” David Nieman, professor of health and exercise science and director of the human performance lab at Appalachian State University told the Los Angeles Times. And regarding the old saying that you can sweat out the flu through intense exercise: “That’s a myth," he says. “A big myth.”
Increasing your body temperature may be dangerous. “If your core temperature is already up and you dare to add to it, you have to keep in mind that viruses like to multiply with higher body temperatures.
“We know that heavy exertion causes a transient downturn in immune function that can last from a few hours to a day,” adds Neiman. Intense workouts can cause stress hormone levels like cortisol to increase, which causes immune cells to function less efficiently. “During this downturn, if you have a virus, it will multiply at a higher rate and make you sick.”
Fever is the limiting factor, adds Dr. Lewis G. Maharam, a sports medicine expert. “The danger is exercising and raising your body temperature internally if you already have a fever because that can make you even sicker,” he says. If you have a fever higher than 101 degrees, sit it out.