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Milk

Much of the vitamin D that our bodies need to build strong bones, defend against heart disease, and—you guessed it—bolster our immune system is produced when the sun's rays interact with our skin cells. But this key vitamin is also found in fortified foods such as milk, orange juice, and breakfast cereal.

Getting your daily dose of vitamin D may keep colds at bay. A 2009 study from Massachusetts General Hospital found that lower vitamin D levels were associated with a greater risk of upper respiratory infections. In 2012, the same researchers found that Vitamin D supplements can help ward off kids' winter colds, as well.

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