Croup, or acute laryngotracheo-bronchitis, is an inflammation of upper and lower respiratory system, including the larynx, due to a viral infection. The inflammation causes a narrowing of the air passages. The most common causative agents are the parainfluenza viruses, especially type 1, the respiratory syncytial viruses (RSV), and influenza A and B viruses.
Croup occurs mainly in children between the ages of three months and three years. In older children and adults, the air passages are too wide and the cartilage in the wall too stiff for swelling or inflammation to cause the walls to collapse. The condition is characterised by fever, cough and breathing difficulty which is accompanied by a harsh croaking noise. Research Croup