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Dark chocolate

Ounce for ounce, pure cocoa contains more of the disease-fighting antioxidants known as polyphenols than most berries—and it's loaded with zinc, to boot.

Too often, however, the nutritional benefits of cocoa are overshadowed by the sugar and saturated fat found in chocolate bars and other treats. To reap the immunity-boosting benefits without the unhealthy extras, stick with bite-sized portions—about one quarter-ounce per day—of dark chocolate with a cocoa content of 70% or higher.

Next: Brazil nuts