Asthma is a disease that affects the breathing passages of the lungs (bronchioles). Asthma is caused by chronic (ongoing, long-term) inflammation of these passages. This makes the breathing passages, or airways, of the person with asthma highly sensitive to various "triggers."

* When the inflammation is "triggered" by any number of external and internal factors, the passages swell and fill with mucus.
* Muscles within the breathing passages contract (bronchospasm), causing even further narrowing of the airways.
* This narrowing makes it difficult for air to be breathed out (exhaled) from the lungs.
* This resistance to exhaling leads to the typical symptoms of an asthma attack.