Childhood asthma. Artwork of a child's respiratory system showing the mucus (yellow) and narrowing of the airways (bronchioles) in paediatric asthma. The mucus is revealed inside the bronchioles by sectioning the right lung (lower centre). An inset (lower right) shows the severe restriction of the air supply that results. In asthma, a trigger such as an allergen causes bronchiole narrowing. It is this that causes the inflammation and production of mucus. This mucus production is also seen in bronchitis, where inflammation usually arises from infection. Treatment is with bronchodilator drugs. Asthma is a chronic disease that usually arises in childhood and improves as the patient matures.