| Sitting at home by the fire in the dead of winter is a comfortable pastime, but it may also be one reason why we get sicker at this time of year. There is a wealth of research that links regular moderate exercise with an improved immune function and reduced susceptibility to the common cold (Exerc Immunol Rev, 1997; 3: 32-52; Sports Med, 1994; 17: 86-107). Analyses of blood from older women who exercise regularly have also shown greater immune activity than the blood of those who do not (Mech Ageing Develop, 1997; 93: 215-22).
However, exercise - like everything else - has a downside. While regular moderate exercise is helpful, bursts of heavy exercise once in a while may be harmful, suppressing immunity for several hours and creating a period of vulnerability when the risk of upper respiratory tract infections is increased (Int J Sports Med, 1994; 15: S131-41). Interestingly, the soldiers most severely affected by the US swine flu outbreak of 1976 were just beginning basic combat training, a time of exceptional exertion (J Infect Dis, 1977; 136 [Suppl]: S363-8). | 
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