Bronchiolitis is inflammation of the bronchioles, the smallest air passages of the lungs.

The term usually refers to acute viral bronchiolitis, a common disease in infancy. This is most commonly caused by respiratory syncytial virus (RSV, also known as human pneumovirus). Other viruses which may cause this illness include metapneumovirus, influenza, parainfluenza , coronavirus and rhinovirus.

In a typical case, an infant under twelve months of age develops cough, wheeze and shortness of breath over one or two days. The diagnosis is made by clinical examination. Chest X-ray is sometimes useful to exclude pneumonia, but not indicated in routine cases. Testing for RSV by nasopharyngeal aspirate is common, but has little effect on management.

The infant may be breathless for several days. After the acute illness, it is common for the airways to remain sensitive for several weeks, leading to recurrent cough and wheeze. There is a possible link with later asthma: possible explanations are that bronchiolitis causes asthma by inducing long term inflammation, or that children who are destined to be asthmatic are more prone to develop bronchiolitis.

There is no effective specific treatment for bronchiolitis. Therapy is principally supportive. Infants who are too short of breath to feed may require nasogastric or intravenous fluids. Oxygen may be required to maintain blood oxygen levels, and in severe cases the infant may need mechanical ventilation.

Ribavirin is an antiviral drug which has some effect in RSV infection, but it is reserved for infants with pre-existing lung, heart or immune disease. Antibiotics are often given in case of bacterial superinfection, but have no effect on the underlying viral infection. Bronchodilator drugs may be effective in some older infants.