Bronchiolitis is an inflammation of the small air passages (bronchioles), usually caused by a virus (respiratory syncytial virus, or RSV, is one such virus). Bronchiolitis usually affects children under the age of 2 years. It is the leading cause of serious lower respiratory illness in infants.

Symptoms of bronchiolitis include labored, rapid breathing with wheezing and a "tight"-sounding cough. If the infection becomes severe, the infant's breathing rate may increase noticeably. If severe difficulty breathing occurs, immediate medical attention is needed.