Cardio and resistance exercise does more than burn off calories, it help circulate your blood, pumping germ-fighting cells through your body and washing away toxins. In fact, 45 minutes of exercise, five days a week has been shown to minimize the amount of sick days by up to 50 percent. But that doesn't mean you have to hit the gym or drop a lot of dough on at-home cardio equipment. Before winter's wrath keeps you inside, take advantage of the clean fall air and take your workouts outside. Take a hike, go for a run, ride a bike, or just play tag with the kids. Just don't overdo it since extended physical activity (like running a marathon) can actually cause too much stress to the body, depleting its resources and weakening your immune system.