COPD is a common lung disease and is the fourth most common cause of death in the United States. Symptoms include coughing, excess mucus production, shortness of breath,and wheezing. There is no cure for COPD, and the disease worsens over time. Treatment can include stopping smoking, taking medications, receiving supplemental oxygen, and in severe cases, undergoing lung transplantation. COPD is usually the result of many years of cigarette smoking, but it remains unknown exactly how cigarette smoking causes this disease.