Bronchiolitis is among the most common and serious lower respiratory tract syndromes that affects young children. In developed countries, the case fatality rate among previously healthy children remains low; in contrast, infants with underlying medical conditions, such as immunodeficiency or chronic lung disease, are at risk of prolonged illness and death. Bronchiolitis is associated with significant morbidity among healthy young children. During the winter season, bronchiolitis is the most common cause of hospitalization among infants. Each year in the United States, approximately 2 per 100,000 infants die as a result of complications associated with bronchiolitis.