Upper respiratory illness (URI) may cause more acute disability among athletes than all other diseases combined. This paper presents the essential epidemiological, infectious, and transmission features of URI. Those who provide health care for athletes must also understand the subsequent implications of URI on sport and exercise participation. While there are reports suggesting that regular, vigorous exercise reduces both the incidence and severity of URI, other research suggests that the immune system may have a distinct level of moderate exercise for optimum health. During winter sports season, URI transmission may escalate. The impact of URI on selected pulmonary, cardiac and skeletal muscle functions may lead to illness complications and protracted courses of URI in athletes. Sport performance during illness may also decline. Athletes should monitor symptoms and training schedules and should rest during a URI.