Research shows that moderate exercise training causes favorable perturbations in immunity and a reduction in upper respiratory illness, including the common cold. During each bout of moderate exercise, an enhanced recirculation of immunoglobulins, neutrophils, and natural killer cells occurs that persists for up to 3-h post-exercise. This exercise-induced surge in immune cells from the innate immune system is transient but improves overall surveillance against pathogens. As moderate exercise continues on a near-daily basis for 12-15 weeks, the number of days with URI is decreased 25-50%. Recent evidence indicates that maintaining leanness and a physically active lifestyle reduces systemic inflammation, an underlying factor in multiple chronic diseases. The Common Cold is Less Common Among the Fit provides an overview of the fact that physically active people tend to have fewer colds.