Bronchiolitis occurs when a virus infects the bronchioles, which are the smallest of the airways branching off the main breathing tubes (bronchi) within your lungs. The viral infection makes the bronchioles swell and become inflamed. Mucus collects in these airways, which can make it difficult for air to flow freely into and out from the lungs.

Most cases of bronchiolitis are caused by the respiratory syncytial virus (RSV). RSV is a common virus which infects just about every child by the age of 2. Seasonal outbreaks of RSV infection occur every winter. Bronchiolitis can also be caused by a variety of other viruses, including those that cause the flu or the common cold.

Bronchiolitis is a contagious condition. You contract the virus just as you would a cold or the flu — through droplets in the air when someone who is sick coughs, sneezes or talks. You can also contract bronchiolitis by touching shared objects — such as utensils, towels or toys — and then touching your eyes, nose or mouth.

May. 07, 2013