Asthma is a chronic lung disease characterized by:

• Airway inflammation
• Airway hyperresponsiveness to various stimuli such as a virus, allergen or exercise
• Airway obstruction or narrowing that is reversible, with treatment or spontaneously

The underlying problem is inflammation as a result of complex interactions among inflammatory cells, mediators and the tissues in the airways. Stimuli activate the release of inflammatory mediators from mast cells, macrophages, eosinophils and other cells in the airways. The mediators signal other inflammatory cells to migrate to the airways where they are activated. This causes injury of the epithelium, prolonged contraction of smooth muscle and secretion of mucus, as well as swelling and changes in the involuntary control of the airway.

Inflamed airways become more narrow and obstructed. Inflammation also causes hyperresponsiveness of the airway, which also results in excessive narrowing of the airways when a stimulus is introduced. Stimuli can include viral respiratory infections, such as colds, which trigger most attacks. Other stimuli are allergens such as pollen or mold, irritants such as tobacco smoke, cold air or exercise. "Trigger" is another term for a stimulus that triggers.

Airway obstruction can develop suddenly or gradually and causes the symptoms associated with asthma: wheezing, coughing, shortness of breath, chest tightness and decreased endurance.