All people with asthma have the same underlying problem: an immune system that overprotects the lungs. Cells that defend the body stay gathered in the airways and produce mucus, a moist substance that keeps the airways free of dust. But if an irritant, such as smoke, pollution, or cold air, comes along, these defense cells overreact: They make too much mucus. The mucus clogs the breathing passages. At the same time, the muscles lining the passages contract, squeezing and narrowing the airways. The result is that less air is able to flow in and out of the lungs.

Doctors aren't sure what causes these cells to stay gathered in a person's airways in the first place, but they do know that the problem tends to run in families. If one of your parents has asthma, the odds that you will have it are higher. You are even more likely to have asthma if both your parents have it. Asthma shows up before the age of 18 for about half of the people who develop the condition. But it can appear at any age, even when a person is in his sixties or older.

We do not know what causes asthma but it tends to run in families that also have hay fever and eczema. You can get asthma at any age. You do not "catch" asthma, but infections can make your asthma worse or cause an asthma attack. Asthma is a chronic condition that you will probably have for the rest of your life.

Asthma is not a problem with breathing in, but with breathing out. During an asthma episode, muscle spasms and swelling bronchial tissues narrow the lungs' tiny airways, which then get clogged with excess mucus. Stale air is trapped in the bottom of the lungs, forcing you to use the top part to gasp for air. Mild and moderate episodes consist of short incidents of breathlessness and wheezing. In severe cases, the lungs' airways become so narrow and clogged that breathing is impossible.

Asthma has no single cause. Episodes can be brought on by a variety of factors working alone or in combination. Allergies are the primary offenders. Between 50% and 90% of people with asthma have allergies. When inhaled, certain substances can trigger the release of histamine and other body chemicals. This sets off an allergic reaction and asthma episodes. Asthma triggers include:

• Immunologic factors
• Viral respiratory infections (particularly in children under 2)
• Sinus infections
• Allergies (most important trigger in children over 2 years of age)
• Exercise-induced asthma (EIA)
• Irritants such as cigarette smoke, burning wood, and air pollution
• Gastroesophageal reflux disease
• Medications
• Food additives and sulfites
• Weather
• Psychologic stressors
• Occupational asthma

Lung infections can also induce asthma. Bronchiolitis, a viral respiratory infection that usually affects children aged 2 years and younger, is a common cause of childhood asthma. Adults may develop asthma as a result of an upper respiratory infection, such as bronchitis. Other asthma triggers include exercise, emotional stress, and environmental stresses such as air pollution.