Bronchiolitis
A viral infection of the smallest airways in infants that causes widespread small-airway obstruction and feeding difficulty; management is supportive because the pathology is mechanical mucosal swelling, not something a drug reverses.
First principles
The disease is small-airway obstruction in an airway that is already tiny
Bronchiolitis is inflammation of the bronchioles, almost always due to respiratory syncytial virus. The virus infects the respiratory epithelium, causing mucosal oedema, sloughing of dead cells and increased mucus. In an infant the bronchioles are only a millimetre or two across, so a small absolute amount of swelling and debris causes a large proportional narrowing (resistance rises with the fourth power of radius). This is why the disease is a problem of the very young: the same inflammation in an adult airway would barely be noticed.
Educational content pending clinical review. Not medical advice.