Bronchiolitis is an infection that affects the lungs and breathing passages; the name “bronchiolitis” means inflammation of the small airways in the lungs. Bronchiolitis is caused by viruses, the most common being respiratory syncytial virus (RSV). The RSV virus is so common that almost all children get RSV by the time they are 2 years of age. For most healthy children, the illness is similar to a cold, with symptoms such as runny nose, mild fever, and cough. However, bronchiolitis can lead to trouble breathing or breathing fast. This can be frightening for parents. Approximately 1% of all healthy babies and 2% to 3% of all high-risk babies are hospitalized for bronchiolitis each year. Most of these hospitalizations are among babies younger than 6 months.