By Markham Heid
December 20, 2018

Some people seem to breeze through cold-and-flu season without so much as a sniffle. What’s their secret?

Regular exercise is a prime candidate. “If you look at all the lifestyle factors that decrease the number of days you suffer from common cold, being a physically active and fit person is the most important,” says David Nieman, a professor of public health and director of the Human Performance Lab at Appalachian State University.

Nieman has spent years examining the effect exercise has on human health and immune function. In one of his studies, he and his colleagues found that 30 minutes of brisk walking increased the circulation of natural killer cells, white blood cells and other immune system warriors.

When these immune cells encounter an illness-causing pathogen, they can kill it very effectively, he says. “But we found that, about three hours after exercise, these immune cells retreat back to the tissues they came from,” he says. In other words, the immune-boosting effects of exercise are fairly short-lived. This is why the “regular” part of regular exercise is crucial. “If you have a housekeeper come in and clean for 30 minutes every day, by end of the month, your house will look a lot better,” he says. “I think the same thing that happens with the immune system and pathogen clearance in the body.”

Nieman says 30 to 60 minutes a day of moderate intensity aerobic exercise—think brisk walking, cycling or easy running—seem to be best when it comes to optimizing immune function. He says weightlifting may prove to be just as effective, but more study is needed. On the other hand, 75 minutes or more of intense exercise may be overdoing it, he says. “When you go that long at a high intensity, stress hormones go way up, and the immune system does not respond well to that.”

Also, while exercise can help prevent illness, it’s not so great at knocking out an existing cold or flu. “Some people think if they get sick, they can sweat it out with exercise,” he says. “But there’s no good data that exercise can be used as therapy.” In fact, research on animals suggests that hard exercise during a cold or flu can make things worse. “Rest is recommended,” he adds.

Apart from exercise, a good night’s sleep is another way to keep your immune system humming.

“We looked at identical twins where one was habitually sleeping an hour or more less than the other,” says Dr. Nathaniel Watson, a professor of neurology and sleep medicine at the University of Washington and first author of a 2017 study on sleep and immune function. “We found that in the shorter-sleeping twin, genetic pathways related to the immune system were suppressed.” He says his study’s findings are in line with other research that has shown sleep-deprived people exposed to viruses are more likely to get sick than well-rested folks.

Exactly how much sleep you need for your immune system to function at its best is tough to gauge. “There’s a lot of individual variability there, so it’s not one-size-fits-all,” Watson says. But getting seven or more hours of sleep a night seems to be a good target for most people. “That’s not seven hours in bed—it’s seven hours of sleep,” he adds.

Finally, a varied and healthy diet is essential. “What we eat fuels our body, and without proper fuel our immune systems don’t work as well,” says Dr. Jason Goldsmith, a postdoctoral researcher at the University of Pennsylvania’s Institute for Immunology. Goldsmith has studied the effect that diets have on the microbiome and immune health. He says most people in the U.S. don’t have to worry about malnutrition. But many people are deficient in certain vitamins and minerals. “In particular, the B vitamins, vitamin C, zinc and vitamin D are important for proper immune function,” he says.

While you could get some or all of these from a pill, he says eating a variety of fruits and vegetables is the better way to go. Along with providing you with the nutrients your body needs, these plant foods also contain soluble fiber, which supports the health of beneficial gut bacteria. These, in turn, seem to promote healthy immune system functioning, he says. (He adds that things are “more complicated” for people with existing medical problems. “We don’t have simple recommendations that can be applied to all patients,” he says, so talk with your doctor.)

The one big exception to this “eat your vitamins” rule is vitamin D, which isn’t easy to find in food. “Vitamin D in particular is important, as deficiency has been associated with both autoimmune diseases and poorer immune function,” Goldsmith says. Taking a vitamin D supplement could reduce your risk for common colds and infections by 10%, research has shown.

So move your body, get some sleep and eat your fruits and veggies. Do that, and friends will be asking you why you never seem to get sick.

Contact us at editors@time.com.

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