Respiratory syncytial virus (RSV) causes acute upper respiratory tract infection in patients of all ages and is one of the most common diseases of childhood. Most infants are infected during the first year of life, with virtually all having been infected at least once by the second birthday. Most RSV-infected infants experience upper respiratory tract symptoms, and 20% to 30% develop lower respiratory tract disease with their first infection. During the first few weeks of life, particularly among preterm infants, infection with RSV may produce minimal respiratory tract signs. Lethargy, irritability, and poor feeding, sometimes accompanied by apneic episodes, may be the presenting manifestations in these infants. Most previously healthy infants who develop RSV bronchiolitis do not require hospitalization, and most who are hospitalized improve with supportive care and are discharged in fewer than 5 days. Characteristics that increase the risk of severe RSV lower respiratory tract illness are preterm birth; cyanotic or complicated congenital heart disease, especially conditions causing pulmonary hypertension; chronic lung disease of prematurity; and immunodeficiency disease or therapy causing immunosuppression at any age. The association between RSV bronchiolitis early in life and subsequent reactive airway disease remains poorly understood. RSV bronchiolitis may be associated with short-term or long-term complications that include recurrent wheezing, reactive airway disease, and abnormalities in pulmonary function. This association may reflect an underlying predisposition to reactive airway disease rather than a direct consequence of RSV infection.

Reinfection with RSV throughout life is common. RSV infection in older children and adults usually manifests as upper respiratory tract illness. More serious disease involving the lower respiratory tract may develop in older children and adults especially in immunocompromised patients, the elderly, and in people with cardiopulmonary disease.