Why do you get every bug that passes through town, while your spouse and friends stay healthy? Blame your immune system, the network of cells and organs that fights off illness (or tries to, anyway).

"How often you get sick is partly genes, plus the bacteria and viruses you're exposed to," says Lisa Cuchara, professor of biomedical science at Quinnipiac University. "But lifestyle is also key: exercise, sleep, and how stressed you are." Read on for how to get your system in fighting shape.

Strategy #1: Do up your diet

"I see a lot of chronic dieters who are low in protein, which your body needs to make white blood cells, the backbone of the immune system," says Dr. Roberta Lee, vice chair of the department of integrative medicine at Beth Israel Medical Center in New York City.
Many protein-rich foods, like lean meat and fish, also provide other immunity-boosting nutrients like iron, zinc, B vitamins, and omega-3 fatty acids. Also essential: Eating a good mix of produce to get an array of nutrients. What to do:

• Pile on the protein. Have some at every meal. A sample day's worth: 8 ounces of yogurt at breakfast, a cup of beans with salad at lunch, a handful of almonds for a snack, and 3 ounces of chicken at dinner.

• Get your daily 8 to 9. Eating this many servings of fruits and veggies may reduce the risk of a cold by about 25 percent, according to research. Aim for at least two colors per meal. One veggie to add: shiitake mushrooms, which may increase natural killer T cells, says Dr. Michael Roizen, chief wellness officer of The Cleveland Clinic.

• Go for fish. Eat salmon, mackerel, or tuna twice a week or more. These fatty varieties are rich in omega-3s, which may reduce your risk of respiratory infection, probably by boosting levels of virus-fighters like helper T cells.
Strategy #2: Pick these pills
The drugstore may be full of so-called immunity boosters, but there's strong evidence for only two of them: vitamin D and probiotics. What to do:

• Shore up your D-fenses. "Vitamin D seems to boost the production of T cells," Dr. Lee says. Since it's hard to get enough D from food, you'll need a supplement. Most doctors recommend 1,000 IU daily.

• Pop a probiotic. This supplement can slash your risk of an upper respiratory infection by about 12 percent. "Probiotics may boost white blood cell activity," says Dr. Melinda Ring, medical director for integrative medicine at Northwestern Memorial Hospital in Chicago. So it's worth trying a daily over-the-counter probiotic (take as directed).

Strategy #3: Hit the gym—but not too hard
In a large 2010 study, those who were active at least five days a week almost halved the length of their colds. Per other research, folks who exercised after getting a flu shot nearly doubled their immune response. Why? Exercise likely sparks a temporary rise in immune cells. What to do:

• Fit in 30 minutes of exercise five days a week. Just don't overdo it: More than 90 minutes of high-intensity exercise can put stress on the body, decreasing your immunity for up to three days.

Strategy #4: Cut back on stress

Pushing yourself physically isn't the only thing that taxes your system. Emotional stress causes your body to release cortisol and adrenaline, which decrease T cells, says Dr. Bruce Rabin, medical director of the University of Pittsburgh's Healthy Lifestyle Program. What to do:

• Laugh. A good giggle can help you stay well. One Loma Linda University study found that laughter boosts virus- and cancer-fighting natural killer cells.

• Pray. Those who regularly attend religious services are about half as likely to have high levels of a protein that increases inflammation. Not a church-goer? Take a few moments to meditate every day.

• Dance. It's a potent relaxer—even if you're just shimmying in your head. One study found that people who merely listened to 50 minutes of dance music had less cortisol and more antibodies. So