The problem is that marathons are held every month of the year, and many athletes’ workout schedules are dictated by the events they compete in, Flynn said. David C. Nieman, director of the Human Performance Laboratory at Appalachian State University in Boone, N.C., and a former president of the International Society of Exercise Immunology, said his research shows that moderate exercise increases the recirculation of important immune cells, especially from bone marrow, the lungs and the spleen. It has the effect of cleaning up the body. “Over time this adds up, much like a housekeeper who comes to your house every day for a half hour. At the end of a month, your house looks a lot better,” Nieman said. But to achieve such beneficial effects, someone has to exercise about five times a week, Nieman said. Nieman agreed with Flynn that grueling endurance training can hurt the immune system: For athletes running 60 miles a week, research shows, their odds of sickness double compared with those who run less. “You can push it too far, and the body says, ‘This good thing is hurting me here,”‘ Nieman said. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! AD Quality Auto 360p 720p 1080p Top articles1/5READ MOREGift Box shows no rust in San Antonio Stakes win at Santa Anita But at a certain point, the physical stress of a long workout undermines the immune system and leaves the endurance athlete even more vulnerable to infection than before a workout. “The general consensus, I believe, is that 30 minutes, three or four times a week, is generally considered to have positive effects,” said Michael Flynn, who studies exercise’s effects on the immune system at Purdue University in West Lafayette. One study showed that jogging about 10 miles a week was beneficial to the body’s defenses, but 20 miles was associated with an increased risk of infection, Flynn said. “There’s pretty good evidence that the intensity of prolonged exercise suppresses certain aspects of the immune system,” Flynn said. “If someone is just doing fitness exercise, it seems like their resistance is better.” So should marathon runners simply scale back their workouts during the cold and flu season? INDIANAPOLIS – Ingrid Ozols Perdue runs several miles a week, lifts weights, cycles, does Pilates and even gets in an occasional kickboxing workout – a regimen she credits with keeping her free of colds and the flu. “I really haven’t been ill,” said the 50-year-old dermatologist from the Indianapolis suburb of Carmel. “I think I’m healthier when I exercise. I think it’s positive for me.” Experts say she’s right – to a point. Research shows moderate amounts of aerobic exercise such as jogging, brisk walking and cycling during the cold and flu season boost the body’s defenses against viruses and bacteria.