Jonas Salk, who developed the polio vaccine, once said, "The mind, in addition to medicine, has powers to turn the immune system around."

He was right. The keys to a healthy immune system include your mind, your body, and a good dose of smart choices about what you feed them both.

First, you want to mind your stress response. Chronic levels of stress hormones such as cortisol alter your gut biome — where 70% to 80% of immune cells reside — making you more vulnerable to infection and mental distress.

Physical activity, meditation, interaction with supportive friends and family, and having a passion all facilitate stress management and a healthier immune system.

The health of your gut biome and its effect on your immune system also depends on your nutritional habits. Being deficient of nutrients such as zinc, selenium, iron, copper, folic acid, and vitamins A, B6, C, D, and E makes you more likely to get hit with bacterial, viral, and fungal infections.

The smart approach: Get these nutrients through fresh, unprocessed foods.

If you're pregnant, breastfeeding, have a chronic illness, or don't eat healthy foods, you may benefit from half a multivitamin/mineral twice daily, as well as omega-3s (900 mg) and vitamin D (1,000 mg).

Other immune boosters include not smoking, seven to nine hours of high-quality sleep nightly, and washing your hands when coming in from outdoors, handling food, after using the bathroom, and before eating.

In addition, maintaining a healthy weight lowers body-wide inflammation; that helps keep your immune system healthy.