The primary ports of viral entry are nose and nasopharynx. The infection spreds and eventually involves the larynx and trachea. the inflammation and oedema of the subglottic larynx and trachea, especially near the cricoid cartilage, are the most clinically significant. Histologically, the involved are is eodematous, with cellular infiltration located in the lamina propia, submucosa and adventitia. the narrowing result in the seal like barky cough, turbulent airflow, stridor and chest wall retraction