Physical activity has long been known to bestow such benefits as helping to maintain a healthy weight and reduce stress, not to mention tightening those abs. Now, a growing body of research is showing that regular exercise—as simple as a brisk 30- to 45-minute walk five times a week—can boost the body's immune system, increasing the circulation of natural killer cells that fight off viruses and bacteria. And exercise has been shown to improve the body's response to the influenza vaccine, making it more effective at keeping the virus at bay.
"No pill or nutritional supplement has the power of near-daily moderate activity in lowering the number of sick days people take," says David Nieman, director of Appalachian State University's Human Performance Lab in Kannapolis, N.C.
via online.wsj.com