Children in this study will attend six study visits over 5 years. At the first visit, parents will review their child's medical and cough history with doctors. Also at this visit, children will undergo a physical exam that will include measures of temperature, blood pressure, heart rate, respiration rate, and oxygen saturation level. Additional procedures will include collection of a respiratory mucus sample or a throat culture, measurement of nasal nitric oxide, collection of blood and urine for specimen banking, a CT scan, and lung function testing. Children younger than 3 years of age will undergo the scan and lung function test under sedation. Children older than 3 years of age will not receive sedation. CT scans will be performed at the initial visit and during the visits 3 and 5 for children older than 3. For children younger than 3 years, chest CT scans will be performed at the initial visit and during visits 4 and 6. Lung function tests and blood and urine collection may be repeated at some of the remaining yearly visits. Between yearly visits, parents will track on a calendar their children's use of oral, inhaled, and intravenous antibiotics.