C. Lower respiratory tract infections (LRI). Lower respiratory tract infections (LRIs) will be defined based on parental responses to questionnaires. LRIs will be defined as a parental report of a physician's diagnosis of pneumonia, bronchiolitis, bronchitis, or croup (laryngotracheobronchitis). LRIs may occur multiple times over a three-year period in a child. Information on repeated episodes of LRI will be employed for rate estimation and comparison.