Bronchial asthma is a chronic inflammation of the bronchi, characterised by recurrent respiratory attacks. During an asthma attack the airway lining thickens, the muscles contract and the mucus obstructs the airways making it difficult to breathe. Airways are particularly susceptible to environmental changes and exposure to irritants, inducing attacks in the patient with the onset of shortness of breath wheezing, and coughing. Among the factors that trigger an asthma attack are infections, exposure to dust mites or pollen, smoke, and air pollution. The disease has various symptoms, and they are often worse at night and early in the morning and the severity of the disease varies from person to person.

Harrison's Principles Of Internal Medicine.