Why don't get sick It’s the height of cold and flu season and many folks have come down with nasty respiratory illnesses after post-holiday stress wreaks havoc with their immune system. But what about those people who seem to sail through the winter months without a sniffle? What’s their secret? According to David Nieman, a professor of public health and director of the Human Performance Lab at Appalachian State University, regular exercise is the secret. “If you look at all the lifestyle factors that decrease the number of days you suffer from the common cold, being a physically active and fit person is the most important,” he says in an article from Time magazine. In one of his studies, he and his colleagues found that 30 minutes of brisk walking increased the circulation of natural killer cells, white cells and other immune system warriors. When these immune cells encounter an illness-causing pathogen, they kill it very effectively, he says. “But we found that, abo...