Croup: An infection of the larynx, trachea, and bronchial tubes that occurs mainly in children. It is usually caused by viruses but sometimes by bacteria. Symptoms include a cough that sounds like a seal's bark and a harsh crowing sound during inhalation. A low-grade fever is common. A major concern with croup is difficulty breathing as the air passages narrow. Treatment may include administration of moist air (as from a humidifier), saltwater nose drops, decongestants and cough suppressants, pain medication, fluids, and, if the infection is bacterial, antibiotics. The breathing of a child with croup should be closely monitored, especially at night, when croup usually gets worse due to prone body position while sleeping. Although most children recover from croup without hospitalization, some may develop life-threatening breathing difficulties. Therefore, close contact with a physician during croup is especially important.