Asthma is a condition that affects the airways — the small tubes that carry air in and out of the lungs.

People with asthma almost always have airways that are sensitive and inflamed. When they come into contact with something that irritates their airways, the passages become narrower, making it more difficult to breathe.

The muscles around the walls of the airways tighten, the lining of the airway becomes inflamed; and sometimes the mucus that cleans and protects the lining of the airways builds up and gets in the way of the air that is trying to reach the lungs.