Although further research is needed with larger study groups and improved study designs, a review of the literature indicates that there are some scientific bases to suggest that heavy or exhaustive exercise may be predisposing to upper respiratory tract illness while moderate amounts may be protective. There exists no convincing evidence suggesting that moderate exercise poses a demonstrable negative influence on the frequency of common respiratory tract infections or adversely influences the course of certain chronic viral illnesses, including viral hepatitis and human immunodeficiency virus infection. Constitutional symptoms secondary to infectious diseases appear to compromise human performance. Exhaustive exercise appears to be a predisposing factor for the development of common respiratory tract infections and may potentially adversely affect the course of certain infectious diseases. Very limited data suggest an enhancement of resistance to respiratory tract infections from moderate exercise training. Although biologically plausible, data to support unequivocal exercise-induced enhancement or suppression of immunologic function are inconclusive at this time.