Causes of bronchiolitis
Bronchiolitis is caused by a virus.
The most common virus that causes bronchiolitis is an airborne virus called respiratory syncytial virus or RSV.
Viruses are spread through the coughs or sneezes of someone who is infected. They can be breathed in from the air or picked up from a surface like skin, toys or door handles.
A third of babies will have had at least one RSV infection by their first birthday.