Asthma is an obstructive lung disease caused by over reaction of the airways to various stimuli. It is classified as an autoimmune disease. In asthmatics, the air passages in the lungs become inflamed, affecting the sensitivity of the nerve endings there so they become easily irritated. During an attack, the lining of the passages swells and produces extra mucus, causing the airways to narrow and reducing the flow of air into and out of the lungs. Asthma is characterized by shortness of breath and wheezing, varying in intensity from person to person. It can occur at any age but generally starts in childhood. A severe attack can be life-threatening.