You might be able to avoid the H1N1 virus, regular flu and the common cold this winter by exercising every day. Exercise gives your immune system a boost, says Dr. David Nieman, director of Appalachian State University's Human Performance Lab in North Carolina. According to Dr. Neiman, two kinds of immune cells circulate more freely in the blood when you excercise, neutralizing pathogens. The effect only lasts for a few hours, but the benefits are cumulative, adding up to a generally stronger immune system. People who walked "briskly" for 45 minutes a day, at least five daysa week for 12 to 15 weeks, reduced their sick days by 25 to 50 percent compared to couch potato controls in his study.