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Croup is an upper respiratory tract infection occurring in infants and toddlers.
Parainfluenza viruses account for the majority of cases. Rarely, other viruses are responsible, for example, RSV, influenza A and B viruses. Unusually, it may be caused by bacteria, for example Staphylococcus aureus.
There is a characteristic inspiratory stridor caused by laryngeal oedema and thick tenacious secretions that block the trachea and airways.
The onset is slower than that of acute epiglottitis and is usually preceded by a coryzal prodrome.