Moderate exercise boosts your immune system; while extreme exercise, such as marathon running, depresses the immune system, moderate exercise, such as walking, may actually prove to benefit it. That’s according to new research from Dr. David C. Nieman, a professor of exercise science at Appalachian State University in North Carolina. Nieman had a group of sedentary women begin walking 45 minutes a day, five days a week, and found that they caught half as many colds as their peers who stayed perched on the sofa. Recently, to see if exercise could stave off aging’s debilitating effects on the immune system, he studied exceptionally active women in their seventies and eighties, who had been walking competitively, for example.