Asthma is due to a decrease in airflow in the major airways of the lungs. It can be caused by muscle spasms in the airways, swelling of cells lining the airways, and excess mucus in the airways. Asthma is one of three respiratory diseases, called "chronic obstructive pulmonary disease," that are often caused by smoking. The other two chronic obstructive pulmonary diseases are chronic bronchitis and emphysema. Chronic bronchitis is caused by small abnormalities in the airways. Emphysema is caused by destruction of the alveolar walls.
Asthma is NOT caused by allergies. An attack is often brought on by a "trigger." A trigger is a factor in the environment that can cause an asthma attack in an asthmatic person, but does not effect non-asthmatics. Allergens are one type of trigger for people who are asthmatic. Exercise is a trigger; for some people, it can trigger an asthma attack.