Croup, also known as croupy cough, is a type of respiratory infection that is usually caused by a virus. The infection leads to swelling inside the trachea, which interferes with normal breathing and produces the classic symptoms of "barking/brassy" cough, inspiratory stridor, and a hoarse voice. Fever and runny nose may also be present. These symptoms may be mild, moderate, or severe. It often starts or is worse at night and normally lasts one to two days.