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
             and may last from a few minutes to several days.
             They may begin at any age but usually occur in
             childhood. In children, asthma often is associated
             with eczema, a skin inflammation that may reflect
             the tendency of the child to develop
             hypersensitivity reactions. The attacks usually
             become less frequent and less severe over the
             years and disappear in about half of all affected
             children before adulthood. In one form of asthma,
             called intrinsic asthma, however, the attacks
             become less frequent and less severe, but
             recovery between them is less complete. The
             bronchi in such patients become chronically
             narrowed, causing a progressive loss of capacity
             for physical exertion. The prevalence of asthma is
             only about 1 or 2 percent worldwide but varies
             greatly from country to country. In the United
             States, asthma affects about 6.9 percent of
             children. Typically, an asthma attack begins within
             minutes after exposure to a triggering agent.
             Symptoms include a sensation of tightness in the
             chest, coughing and wheezing, and difficulty in
             breathing. Persons having attacks usually find it
             more difficult to exhale then inhale, which causes
             overinflation of the chest and impaired lung
             functions. The breathing difficulty is alleviated
             somewhat by leaning forward and supporting the
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