
Bronchiolitis is usually caused by a viral infection, most commonly respiratory syncytial virus (
RSV).
RSV infections are responsible for more than half of all cases of bronchiolitis and are most widespread in the winter and early spring.
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It sounds like the relationship of bronchiolitis and asthma is still being studied. In the meantime, I would work to ease the other factors, such as exposure to second-hand cigarette smoke, that he might be exposed to. Good luck!