Bronchiolitis is almost always caused by a viral infection. In most cases, the respiratory syncytial virus (RSV) is responsible.
RSV is a very common virus and almost all children are infected with it by the time they're 2 years old.
Viruses are spread when an infected person coughs or sneezes.
Tiny droplets of liquid can be breathed in directly from the air or picked up from a surface they have landed on, such as a toy or table.
For example, your child can become infected after touching a toy that has the virus on it and then touching their eyes, mouth or nose.
RSV can survive on a surface for up to 24 hours.
An infected child can remain infectious for up to 3 weeks, even after their symptoms have disappeared.
Once you become infected, the virus enters the respiratory system through the windpipe (trachea).
The virus makes its way down to the smallest airways in the lungs (the bronchioles).
The infection causes the bronchioles to become inflamed (swollen) and increases the production of mucus.
The mucus and swollen bronchioles can block the airways, making breathing difficult.
As babies and young children have small, underdeveloped airways, they're more likely to get bronchiolitis.
Bronchiolitis is very common in infants and is usually mild.
Several things can increase a child's likelihood of developing the infection.
There are also a number of factors that can increase the risk of a child developing more severe bronchiolitis.