Asthma, an incurable disease that afflicts nearly 10 million Americans and kills between 2,000 and 4,000 a year, is more easily described than defined.

It makes breathing difficult. The wheezing, spasms and shortness of breath during an asthma attack come from a chronic but reversible narrowing of the bronchial tubes, the airways leading to the lungs.

The underlying cause of asthma is still not fully understood. Many factors -- including respiratory infections, aspirin, exercise and emotional stress -- can trigger an asthma attack in a susceptible person.

In nearly half the cases, asthma is related to an allergic reaction to pollen, dust, animal dander, molds or some kind of food. Environmental irritants such as cigarette smoke, cooking odors and aerosol sprays can worsen an asthma attack.

One-third of the people with asthma are children, although about half of those will outgrow the illness, for unknown reasons. In general, asthma attacks are less severe, less frequent and less lasting in people who develop the illness at an early age. The exception is asthma in children under 2.

Since asthma can have so many causes, no single drug works well in every case. The disease is treated with a variety of drugs, often in combination. These include bronchodilators (which widen the airways, relieving bronchial constriction), methylxanthines, steroids and cromolyn (which helps prevent allergic reactions in the lungs and is especially useful in people susceptible to exercise-induced asthma).

Warning: Many of these drugs have serious side effects. Their use should be carefully monitored by a doctor.

In cases of asthma triggered by allergic reactions, the best approach is to pinpoint the cause of the allergy and avoid it if possible.