Inflammation affecting the finest bronchial tubes. Major epidemics occur every winter in Northern Europe in babies under 18 months, mostly due to respiratory syncytial virus (RSV). The illness starts with symptoms of a cold which progress to rapid breathing, difficulty in feeding and – in the most severe cases – a poor skin colour due to a diminished oxygen transfer from lungs to blood. While the condition is mild in most infants, a few need admission to hospital for monitoring in case they might need artificial ventilation for a time. In some bronchiolitis may represent the first episode of what will eventually be ASTHMA.