text: Croup: laryngotracheobronchitis, viral croup, spasmodic croup, laryngismus stridulus\n\nCauses:\n\n    * Parainfluenza virus type I, which can be recovered in 60% of cases, causes most cases of croup.\n\n    * Parainfluenza types II, III, and IV; influenza A and B; adenovirus; respiratory syncytial virus; herpes simplex virus; measles; rhinovirus; coxsackievirus A and B; echovirus all can cause mild sporadic croup.\n\n    * Influenza A has been implicated in children with severe respiratory compromise.\n\n    * In 1 case, Mycoplasma pneumoniae has been recovered.\n\n    * Bacterial superinfection or bacterial tracheitis is uncommon. Staphylococcus aureus (most common), Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis cause bacterial superinfection or bacterial tracheitis. Group A streptococcus, viridans streptococci, gram-negative enteric bacteria, and anaerobes also have been implicated. When S aureus invades the trachea, it causes mucosal ulceration and necrosis, and the trachea is covered with a thick purulent exudate. This increases luminal narrowing and exacerbates the respiratory compromise.