Bronchiolitis is an acute viral infection of the small airways of the lungs (the bronchioles). It typically presents with airways obstruction. Bronchiolitis is usually caused by the respiratory syncytial virus (RSV) and occurs commonly in children under one year of age. When severe it can be life threatening. Bronchiolitis usually occurs in winter and follows a few days after the onset of a common cold. The small airways become inflamed and narrowed. Secondary bacterial infection may occur.