Bronchiolitis (primarily RSV) involves viral invasion of bronchiolar epithelial cells causing necrosis, edema, mucus hypersecretion, and small airway obstruction. In infants with bronchiolar lumens less than 1 mm, even 1 mm of edema reduces cross-sectional area by 75%, creating a ball-valve mechanism with air trapping, hyperinflation, and hypoxemia. Croup (laryngotracheobronchitis, primarily parainfluenza virus) involves subglottic inflammation below the cricoid cartilage, the narrowest portion of the pediatric airway. Subglottic edema of 1 mm reduces the infant airway cross-sectional area by approximately 60%, producing the classic triad of barking (seal-like) cough, inspiratory stridor, and hoarseness.