Asthma is a disease of the respiratory system in which the airways constrict, become inflamed, and are lined with excessive amounts of mucus, often in response to one or more "triggers," such as exposure to an environmental stimulant (or allergen), cold air, exercise, or emotional stress. In children, the most common triggers are viral illnesses such as those that cause the common cold[citation needed]. This airway narrowing causes symptoms such as wheezing, shortness of breath, chest ...
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