Sometimes the airway overreaction causes swelling throughout the entire bronchial tube. The opening in the center of the tube then becomes smaller, and breathing becomes more difficult. When the muscles relax, the bronchial tubes return to a near-normal size. Asthma symptoms then resolve, and breathing becomes easier. Others have just a few or only one.
Symptoms often get worse in the early morning and during the night.
Asthma is caused by an immune system response. The immune system overreacts to triggers and causes the airways to become inflamed and tight. Asthma is more common in children who were underweight at birth and those who live in polluted areas. It is more prevalent in children who were given formula than in those who were breast-fed as infants. Children who attended day care as infants are only half as likely to develop asthma as other children.