Asthma is a chronic disease that affects the airways and lungs. According to the World Health Organization (WHO), approximately 300 million people worldwide and 14–17 million people in the United States are affected by asthma. Asthma is the most common chronic disease in children. Severe asthma is a serious health concern that can lead to respiratory failure. Each year in the United States, as many as 470,000 people seek hospital treatment for asthma-related symptoms and the disease causes about 5000 deaths. The cause for this condition is still unknown. Inflammation of the bronchial airways causes them to become constricted and narrowed. Narrowing of the airways, called bronchoconstriction, produces shortness of breath, tightness in the chest, cough and wheezing. Except in severe cases, symptoms of asthma are occasional. The duration and severity of asthma symptoms vary greatly from time to time and from patient to patient. The symptoms may be intermittent, and they can last just a few minutes or days. In severe cases, asthma symptoms may be constant and persistent.