There is increasing evidence to suggest that both acute bouts of strenuous exercise and prolonged periods of high-load exercise are associated with immune suppression and increased incidence of upper respiratory infections (URIs) in well-trained athletes. In contrast, moderate physical activity seems to increase immune function, which has been associated with a lower incidence of URIs when compared with sedentary individuals. Hence, the relationship between exercise workload and the incidence of URIs has been proposed by David Nieman to be a J-shaped curve.

Moreover, mucosal immunity, particularly salivary immunoglobulin A (s-IgA) has been implicated as a major risk factor for URIs. s-IgA is the predominant immunoglobulin in mucosal secretions and provides the first line of defense against pathogens and antigens that present at the mucosal surface. s-IgA ...