Asthma is the leading cause of chronic illness in children. It affects about 7 million children in the United States and, for unknown reasons, is steadily increasing. Asthma can begin at any age (even in the very elderly), but most children have their first symptoms by age 5.No one really knows the exact reasons why more and more children are developing asthma. Some experts suggest that children spend too much time indoors and are exposed to more and more dust, air pollution, and secondhand smoke. Some suspect that children are not exposed to enough childhood illnesses to direct the attention of their immune system to bacteria and viruses.