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The effect of an 18.5-week infantry training program on health status was studied in 23 male military personnel (aged 22.0 +/- 0.5 years, mean +/- SE). Aerobic power, body composition, and immune function (including natural killer cell activity, mitogen-stimulated lymphocyte proliferation, in vivo cell-mediated immunity, and secretory immunoglobulin A levels) were measured in subjects at the beginning and end of the course. Subjects self-reported their symptoms of sickness in health logs using a precoded checklist. Data from this study indicate that subjects became leaner and maintained, but did not increase, their aerobic fitness by the end of the course. Cell function was enhanced significantly; however, in vivo cell-mediated immunity remained the same, and levels of secretory immunoglobulin A were lower by the end of the course. The incidence of infection remained stable throughout the course. These results indicate that the current pattern of infantry training does not have an adverse effect on the health status of recruits.

Coaches and athletic team physicians have provided anecdotal information and case studies to support their beliefs that athletes may be unusually prone to illness during strenuous training or competition. Many athletes, in contrast, believe that physical activity improves their resistance to infectious disease. However, it is generally agreed that the stress of competition may make athletes temporarily more susceptible to infectious illness. A review of the literature shows that upper respiratory tract infections and skin infections are more prevalent in top level athletes than in the general population, particularly during periods of intensive training. Exercise induced changes occur in both the innate and adaptive components of the immune system; however, the relative importance of each component is unknown. Strenuous exertion and contact sports may compromise host defence both by reducing physical protection and by impairing immunosurveillance. Skin lacerations, vigorous sweating and maceration of the dermis impair the defence normally provided by the skin surface. In addition, adverse changes in soluble and cellular components of the immune system can increase susceptibility to infection. Persistence with strenuous training during an infectious illness can have deleterious effects; not only is athletic performance impaired, but the severity of the disease process can be augmented.