SummaryViruses are ubiquitous and cause numerous infections in humans. These may vary from asymptomatic infection to severe debilitating illness. Viruses enter the host cells to replicate, using host synthetic mechanisms, and, thus, are resistant to conventional antibiotics. The human body responds to viral infection by synthesising specific antibody which can be used to aid diagnosis. Infectious mononucleosis (glandular fever) commonly affects the 15 to 30 years age group. It may produce severe debility which may last a month or more. Coxsackie virus infection can produce symptoms of the common cold but may also invade heart muscle and produce myocarditis, a potentially serious disease. Other viruses also produce a wide spectrum of disease.Recent evidence has shown that people undergoing severe mental or physical stress may have reduced immunity to viral infections. There are risks associated with strenuous physical activity during the acute phase of viral infection, and there are reports of sudden death and serious complications occurring in previously fit young adults who undertake vigorous exercise when in the acute phase of a viral illness. Abnormalities of skeletal muscle have been demonstrated in patients with viral infection and this may explain the loss of performance experienced by athletes after upper respiratory tract infection.As a general rule, for all but mild common colds, it is advised that the athlete avoids hard training for the first month after infection.