Croup (acute laryngotracheobronchitis) is a viral infection causing inflammation and edema of the subglottic airway, primarily affecting children 6 months to 3 years (peak incidence 1-2 years). Parainfluenza virus types 1 and 3 account for ~75% of cases; other causes include RSV, influenza, adenovirus, and human metapneumovirus. The subglottic trachea is the narrowest part of the pediatric airway (cricoid cartilage is a complete ring that cannot expand), making children uniquely susceptible to symptomatic obstruction from even mild edema. One millimeter of circumferential edema reduces the cross-sectional area of the infant subglottic airway by ~60% (Poiseuille law: airflow resistance is inversely proportional to radius to the fourth power). This produces the characteristic barking/seal-like cough, inspiratory stridor, and hoarseness.