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Causes
By Mayo Clinic staff
The windpipe (trachea) is the main airway to your lungs. Within your lungs, the trachea branches off into two main breathing tubes called bronchi, one into your left lung and one into your right. Within each lung, the bronchi branch off into smaller and smaller air-tube passageways, distributing air throughout your lungs. The smallest of these airways are called bronchioles.
Bronchiolitis occurs when a virus enters the respiratory system and makes its way to the bronchioles, causing them to become inflamed and swollen. As a result, mucus often collects in these airways, which can make it difficult for air to flow freely through your lungs.
In older children and adults, the resulting signs and symptoms are generally mild. But an infant's bronchioles are much narrower than are an adult's and are more easily blocked, leading to greater difficulty breathing.
The respiratory syncytial virus (RSV), a common virus, causes most childhood bronchiolitis cases. The rest are caused by other infectious agents, including viruses that cause the flu or the common cold. Severe cases of bronchiolitis may reflect multiple infections, such as a combination of RSV and metapneumovirus.
Bronchiolitis is a contagious condition. You contract the infectious virus just like you would a cold or the flu — by inhaling airborne droplets of infected mucus or other respiratory secretions or by touching objects contaminated by these secretions and then touching your eyes or the inside of your nose or mouth. There is no vaccine for most of these viruses.
