Bronchiolitis - Causes of bronchiolitis

Bronchiolitis is almost always caused by a viral infection. In the majority of cases the virus responsible is the 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.

How the infection is spread

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 up to 24 hours.

An infected child can remain infectious for up to three weeks, even after the symptoms have gone.

How it affects the lungs

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 it difficult to breathe. As babies and young children have small, underdeveloped airways, they are more likely to get bronchiolitis.

Who's most at risk?

Bronchiolitis is very common in infants and is usually mild. However, there are several things that can increase your chances of developing the condition. These include:

• 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

There are also a number of things that can increase the risk of a child developing more severe bronchiolitis. 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)