Bronchiolitis is almost always caused by a viral infection. In 75% of cases, the virus responsible is the respiratory syncytial virus (RSV).
Respiratory syncytial virus (RSV)
RSV is a very common virus. Almost all children are infected with RSV by the time they are two years old. In older children and adults, RSV may cause a cough or cold, but in young children it can cause bronchiolitis.
Other viruses that may cause bronchiolitis include:
- parainfluenza virus
- influenza (flu) virus
- adenovirus
- rhinovirus
Catching a virus
Viruses are spread through tiny droplets of liquid from the coughs or sneezes of someone who is infected. The droplets can be breathed in directly from the air or picked up from a surface that they have landed on, such as a toy or table.
For example, your child can become infected if they touch a toy that has the virus on it and then touch their eyes, mouth or nose. RSV can survive on a surface for 6-12 hours.
Children can be infectious for up to three weeks after having the condition, even after their infection has cleared up.
The respiratory system
Once the virus has been caught, it 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 it difficult to breathe. As babies and young children have small, underdeveloped airways, they are more likely to get bronchiolitis.
Who’s at risk?
Bronchiolitis is very common in infants and is usually mild. However, there are several things that can make bronchiolitis more serious. These include:
- being under two months of age
- having congenital heart disease (a birth defect that affects the heart)
- being born prematurely (before week 37 of pregnancy)
- having chronic lung disease of prematurity (when injury to the lungs causes long-term respiratory problems in premature babies)
- being breastfed for less than two months or not at all
- being exposed to smoke, for example if parents smoke
- having brothers or sisters who attend school or nursery (as they are more likely to come into contact with a virus and pass it on)
