Asthma is a chronic disorder of the lungs in which inflamed airways are prone to constrict, causing episodes of breathlessness, wheezing, coughing, and chest tightness that range in severity from mild to life-threatening. Inflamed airways become hypersensitive to a variety of stimuli, including dust mites, animal dander, pollen, air pollution, cigarette smoke, medications, weather conditions, and exercise. Stress can exacerbate symptoms.