Bronchiolitis, a lower respiratory tract illness most often caused by respiratory syncytial virus, generally affects children under two years of age, commonly during the winter months. Necrosis of epithelial cells in the small airways leads to inflammation and airway obstruction, causing decreased oxygen saturation, with cough and wheezing. Hospital admission should be considered for children with pulse oximetry levels less than 95 percent at rest. Treatment consists of humidified oxygen, intravenous hydration and administration of nebulized albuterol. Infants with mild disease who are identified early in the course of illness should be reevaluated in 24 hours. Infants with congenital heart disease, bronchopulmonary dysplasia or a history of prematurity, who are at high risk for severe disease, should be treated with ribavirin.