Moderate exercise helps to protect us from influenza, but strenuous, exhaustive exercise increases susceptibility to influenza and other respiratory infections. Hong Kong researchers discovered individuals that never or seldom exercise increased their risk of dying from influenza from between 5½ to 8 times. In contrast, those who exercised frequently decreased their risk by 4 to nearly 6½ times.
In a study done by scientists at Iowa State University, the subjects who exercised vigorously following a flu vaccine had a higher concentration of both IgG and IgM, suggesting a greater degree of protection. Our data on symptom incidence is inconsistent with this possibility.
Moderate exercise also increases antibody production, improves T-lymphocyte function in the elderly, and slows down the aging of the immune system. This means that with moderate exercise even older people can develop an increased resistance to viral infections.
Wong C.M., et al, Is exercise protective against influenza-associated mortality? PLoS ONE, 3(5):e2108, 2008.
Nehisen-Cannarella, S.L., et al, The effects of moderate exercise training on immune response. Med Sci Sports Exerc, 23(1):64-70, 1991.