I get 7 to 8 hours of sleep most nights, I make it to my favorite workout classes regularly, and I eat all of my servings of fruits and vegetables in every day (okay, I leave room for wine and dessert too).

Yet every year, I manage to come down with several colds, bouts of strep throat, and stomach bugs. I even got shingles a few years ago. My burning question: Why is my immune system so weak?

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The short answer: It’s possible that some people have naturally stronger immune systems than others. “I have some perfectly healthy patients who get five to six upper respiratory infections—aka, the common cold, or more rarely, full-blown influenza—a year, and others with the same health profile who hardly ever get sick,” says Holly Phillips, MD, an internal medicine physician in New York City and author of The Exhaustion Breakthrough. “It doesn’t seem fair, and honestly it’s not.”

On the flip side, some individuals are also thought to be especially resistant to certain bacterial and viral infections. “Immunologists refer to them as having ‘super-immunity,’ and their genetic makeup is the focus of an entire field of research,” Dr. Phillips says.

Your immunity is determined by several factors. Genetics play a large role, says Paolo Boffetta, MD, professor of medicine, hematology, and medical oncology at Icahn School of Medicine at Mount Sinai Hospital in New York City. “Immunological genes are many and complex, and your immune response depends on their combined performance,” Dr. Boffetta explains.

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Experts also believe that you build up immunity while you are young, or possibly even before you are born. “There are factors during infancy and childhood, and perhaps also in utero, that contribute to the development of the immune system, but they are not fully understood,” Dr. Boffetta says. What’s more, environmental factors, such as pollutants and toxins, may also weaken your immune system over time.

One standout theory for why some people may just be more susceptible to illness is the hygiene hypothesis—the idea that the more bacteria and viruses you come into contact with as a kid, the “smarter” your immune system becomes. The thinking is “keeping kids extra clean and smothering them in hand sanitizer may cause more harm than good in the long run,” Dr. Phillips explains.

On top of that, lifestyle factors, including stress, obesity, heavy alcohol drinking, and poor hygiene, can all weaken your immune response and lead to more frequent infections, Dr. Boffetta notes.

How to strengthen your immune system

Just because your immunity is not something you can entirely control doesn’t mean you should abandon your early bedtime or start loading up on junk food. Nutrients from fruits and veggies, like zinc, iron, and vitamin C, are essential for a healthy immune system, Dr. Phillips says, as are omega-3 fatty acids. “Omega-3s found in fatty fish, like salmon, also encourage the production of lymphocytes—immune cells that are the front line of defense against infections,” she explains. And it’s better to get your immune-protective nutrients through food than through supplements, she adds.

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Getting plenty of sleep and exercise does in fact help boost your immune system—whether it feels like these healthy habits are working in your favor or not. Exercise improves circulation; increased circulation boosts the production of germ-fighting antibodies, Dr. Phillips explains. “Exercise also lessens stress, which helps your immune system function at its best,” she says.

Sleep is a health-restorative time for the body. “When you sleep, your body releases immune proteins called cytokines, which help fight infections and control the body’s response to stress,” Dr. Phillips says. “So a lack of sleep can lead to an under-production of cytokines and other protective immune cells, leaving you more vulnerable to infection.”

via Why Some People With Healthy Habits Still Get Sick A Lot