Interaction between infection and exercise with special reference to myocarditis and the increased frequency of sudden deaths among young Swedish orienteers 1979-92.

Infection and fever evoke a cytokine-mediated host response resulting in negative nitrogen balance, muscle protein degradation, which includes the skeletal muscles as well as the heart muscle, and deteriorated muscle function. Physical training has an opposite effect. Moderate physical training also stimulates the immune system, whereas exhaustive and longlasting exercise is followed by a temporary immunodeficiency and an increased susceptibility to respiratory tract infections. Exercise in the acute phase of an infection may promote complications including myocarditis. Exercise in myocarditis is associated with increased organism-associated as well as immune mediated tissue damage. An increased sudden death (SUD) rate among young Swedish male orienteers existed in 1979-92, suggesting (a) common underlying cause(s). Myocarditis was one of the most conspicuous histopathological features. Chlamydia pneumoniae, or a similar organism cross reacting in diagnostic tests, is hypothesized to be a factor causing this increased death rate. High frequency of intense exercise sessions, which was a common practice among the deceased, may have been immunosuppressive, promoting the development of severe myocardial disease.