Call 911 in an emergency, and the dispatcher urgently asks, "Where are you?"

When a virus attacks, your immune system wants the same info - which of the trillions of cells in your body is in trouble? Enter arginine, a nutrient that knows how to yell "SOS."

This amino acid hangs a "Help! Over here!" sign on newly infected cells. That attracts virus fighters with the firepower to knock out a cold or flu. In one new study, kids prone to common colds who got extra arginine had fewer sniffles. In another, arginine helped cells beat back infections better.

There's no official target for how much arginine you need in a day, though in addition to fighting viruses, your body uses arginine to manufacture nitric oxide, which keeps arteries young and supple. Being on the low end isn't a good idea. It may boost bodywide inflammation, encouraging buildups of heart-threatening plaque in artery walls.

No need to take a supplement. Plenty of delicious, healthy foods - shrimp, crab, lobster, orange roughy, tilapia, canned light tuna packed in water; chicken and turkey breasts - are full of arginine. And guess what? So are many of your favorite snacks: raisins, nuts and seeds, sugar-free gelatin, even chocolate. (Make that 70 percent cocoa dark chocolate, and you'll encourage blood flow and coddle your arteries.) It doesn't take a nutritionist to figure out these should be your go-to munchies during cold and flu season.

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