Asthma is a chronic inflammatory disease of the lungs. People with asthma have airways that are hyperresponsive, meaning that the airways react to asthma triggers such as colds, cigarette smoke and exercise faster and more intensely than people whose airways are normal.
Asthma is a complex disease. Health professionals do not know for certain what causes asthma, but studies have shown that both genetics and the environment can affect your risk of getting asthma. Your family history can help you and your doctor understand what your risk may be for asthma.
National Heart, Lung and Blood Institute Asthma link