Asthma is a condition characterized by unpredictable and disabling shortness of breath. It features episodic attacks of bronchospasm (prolonged contractions of the bronchial smooth muscle), and is a complex disorder involving biochemical, autonomic, immunologic, infectious, endocrine, and psychological factors to varying degrees in different individuals.

Asthma occurs in families suggesting that there is a genetic predisposition for the disorder, although the exact mode of genetic transmission remains unclear. The environment appears to play an important role in the expression of the disorder. For example, asthma can develop when predisposed individuals become infected with viruses or are exposed to allergens or pollutants. On occasion foods or drugs may precipitate an attack. Psychological factors have been investigated but have yet to be identified with any specificity.

The severity of asthma attacks varies among individuals, over time, and with the degree of exposure to the triggering factors. Approximately half of all...