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 two years old. In older children and adults, RSV may cause a cough or cold, but in young children it can cause bronchiolitis.
Useful links and resources
Feeding your baby/infant when they have RSV resulting in brochiolotis