Why You Shouldn't Exercise If You’re Feeling Sick

If you want to protect yourself from colds and flu, regular exercise may be the ultimate immunity-booster. Studies have shown that moderate aerobic exercise—around 30 to 45 minutes a day of activities like walking, biking or running—can more than halve your risk for respiratory infections and other common winter maladies.

There’s some evidence that very intense exercise—running a marathon, say—can briefly suppress your immune function, says Dr.