A common disorder in which chronic inflammation of the bronchial tubes (bronchi) makes them swell, narrowing the airways and making it difficult to breathe. Asthma involves only the bronchial tubes and does not affect the air sacs or the lung tissue itself. Airway narrowing in asthma is due to three major processes acting on the bronchi: inflammation (see above), spasm (bronchospasm), and hyperactivity (over-reaction or allergy to some substance)