Causes

Bronchiolitis occurs when a virus infects the bronchioles, which are the smallest airways in your lungs. The infection makes the bronchioles swell and become inflamed. Mucus collects in these airways, which makes it difficult for air to flow freely in and out of the lungs.

Most cases of bronchiolitis are caused by the respiratory syncytial virus (RSV). RSV is a common virus that infects just about every child by 2 years of age. Outbreaks of RSV infection occur every winter, and individuals can be reinfected, as previous infection does not appear to cause lasting immunity. Bronchiolitis also can be caused by other viruses, including those that cause the flu or the common cold.

The viruses that cause bronchiolitis are easily spread. You can contract them through droplets in the air when someone who is sick coughs, sneezes or talks. You can also get them by touching shared objects — such as utensils, towels or toys — and then touching your eyes, nose or mouth.