Asthma is a problem with the lungs that causes the airways to swell and produce lots of thick mucus. The muscles surrounding the airways also tend to tighten, which makes the already clogged airways even narrower. This can lead a person to cough, wheeze, and have trouble breathing. People with asthma may be sensitive to certain things, such as viruses, allergies, smoke, and even exercise. Doctors call asthma a chronic condition. This means that people who have asthma will always have the condition, although they may not always have symptoms.