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 affects infants. It is the leading cause of serious lower respiratory illness.

Symptoms of bronchiolitis include labored, rapid breathing with wheezing and a cough that sounds "tight." If the infection becomes severe, the infant's breathing rate may increase noticeably.

If severe difficulty breathing occurs, immediate medical attention is needed.