We've all heard that exercise can help stave off illness by offering an immune system boost, but what kind of activity is best? According to one expert who surveyed the research, moderate exercise -- things like taking a brisk walk or playing touch football with friends -- can reduce our risk of getting colds and flu viruses. But in a case of "less is more," the same is not true of prolonged, intensive training, like the kind undertaken by marathon runners and elite athletes. Unlike an average workout, a marathon can actually increase the likelihood that an athlete will get sick. A totally sedentary person is likely to contract a yearly average of two to three upper respiratory tract infections (URTIs) -- the medical term for viral infections of the ear, nose and throat, like colds, flu and sinus infections. But a moderately active person can expect to reduce that rate by almost a third, according to Mike Gleeson, a professor of exercise biochemistry at Loughborough University in Leicestershire, U.K. By contrast, an elite endurance athlete who completes intensive training can expect two to six times as many URTIs during a year.