Story provided by Quartz – 

Last Saturday, I woke up with a sore throat. I had recently had a head cold, though, so I figured it was nothing to worry about. I popped some ibuprofen and completed a 14-mile training run as planned.

Exercising when you’re well increases your blood flow and circulates more immune cells throughout your body, where they can be on the lookout for potential pathogens. Exercise also reduces stress, which can impede your body’s ability to fight off disease.

But when you’re sick, working out has diminishing returns. David Nieman, an exercise physiologist at Appalachian State University, developed the J-curve (paywall) almost 20 years ago to describe the immune system’s relationship to exercise. The model suggests that when you’re well or have a minor upper-respiratory infection—like a head cold or sore throat—moderate amounts of exercise won’t hurt you, and may even help your immune cells reach all the areas they need to in your body (think the downward curve of the “J”). But, if you overtrain or exercise too intently while you’re sick, you’re likely to only prolong your symptoms, or make yourself susceptible to picking up something worse (think the upper-increase tail of the “J”).

Nieman says that when you do physically push yourself—especially while sick—you suppress your defenses. “Stress hormones [from physical exertion] down-regulate the function of some of the immune cells, and that results in the open window where viruses can multiply at a higher rate than normal.”

Missimer says your best guide is “listening to your body.” Missmer was diagnosed with stage-3 liposarcoma, a rare form of cancer, last year. She had exercised her entire life, and continued to do so during treatment. “If there was a day where I felt really exhausted and I felt I just couldn’t do it, I wouldn’t do it that day or would do something very light,” she says. “But then on a day where I felt pretty good, then I might push it a little more.”