Asthma is a chronic lung disease that inflames and narrows the airways. This makes the airways swollen and very sensitive. They tend to react strongly to certain substances that are breathed in.
The exact cause of asthma isn’t known. Researchers think a combination of factors (family genes and certain environmental exposures) interact to cause asthma. Different factors may be more likely to cause asthma in some people than in others.
Asthma affects people of all ages, but it most often starts in childhood. In the United States, more than 22 million people are known to have asthma. Nearly 6 million of these people are children. Among children, more boys have asthma than girls. But among adults, more women have the disease than men. Its not clear whether or how gender and sex hormones play a role in causing asthma. Most, but not all, people who have asthma have allergies.
— Source: National Heart, Lung and Blood Institute