The susceptibility to upper respiratory tract infections (URTIs) after physical exercise has been described with a J-shaped curve, suggesting protection from infections with moderate exercise and increased risk for URTI's in elite athletes. Several factors such as time of inoculation, previous infections, pathogen exposure, other stressors than exercise etc. can influence infection outcome. Observed infections in athletes can, therefore, be either the result of increased susceptibility to a novel pathogen, or more severe symptoms of an already established infection. Moreover, the definitions of "strenuous" exercise and "elite" athletes are equivocal, making comparisons between studies difficult. Because absence of infections is inevitable to become and maintain status as an elite athlete, it is suggested that there is an S-shaped relationship between exercise load and risk of infections. To become an elite athlete one has to possess state-of-the-art physique, including an immune system able to withstand infections even during severe physiological and psychological stress.