Bronchiolitis is an infection of the small airways. It is also the most common manifestation of acute lower respiratory infection (ALRI) in early infancy, and is the leading cause of global child mortality. Viral bronchiolitis is currently the most common reason for pediatric hospital admission in the US, accounting for almost 20% of all-cause infant hospitalizations. Viral etiology is the main cause, and among the respiratory viruses, respiratory syncytial virus (RSV) is believed to be the most important viral pathogen causing ALRI in young children.

Worldwide, it is estimated that 60,000 to 199,000 children younger than 5 years die yearly from RSV-associated ALRI, with 99% of these deaths occurring in developing countries. The disease is common mainly in the first year of life. The clinical signs and symptoms are consistent with hypoxia, difficulty breathing, coryza, poor feeding, cough, wheeze and crepitations on auscultation, and in some cases respiratory failure.