Asthma {az'-muh} Asthma is a respiratory disorder marked by breathing difficulty caused by temporary narrowing of the bronchi, the airways branching from the trachea to the lungs. Attacks usually are brought on by allergic reaction to ANTIGENS such as grass and tree pollens, mold spores, fungi, animal dander, and certain foods but may also be caused by chemical irritants in the atmosphere or by infections of the respiratory tract. Susceptibility to an asthma attack is based on hyperactivity of the bronchial muscles, which constrict on exposure to one or another of these agents. Episodes of asthma vary widely in severity