Asthma (known as wheezy bronchitis decades ago) is a temporary narrowing of, and excess production of phlegm in, the small airways (bronchioles) through which air flows into and out of the lungs. The narrowing is caused by a spasm in the tiny muscles that surround the bronchioles. One in ten people in western countries suffers from some degree of asthma, but it is uncommon in developing countries.

Illustration showing bronchial asthma with comparison of normal airway and affected airway

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