What is croup? Croup is a throat inflammation that affects the larynx (voice box) and trachea (windpipe). It is characterized by a barking cough and high-pitched sounds made during inhalation (stridor). The cough tends to get worse at night. Croup is most commonly seen in young children, especially children between the ages of three months and five years. Boys are more often affected. Croup is usually a viral infection that is most commonly caused by a parainfluenza virus, which is transmitted person-to-person by airborne droplets or contact (touch). Other viruses, such as adenovirus, the flu, and measles virus, can also cause croup. The reason you hear a high-pitched sound during inhalation is due to swelling and inflammation of the child’s airway. Left untreated or poorly controlled, croup may advance with continued swelling that can make breathing an exhausting effort, and your child may feel too tired to eat, drink or cough. Seek immediate medical care (call 911) if your child has signs of croup and you suspect it may be caused by a recent insect sting or from inhaling a foreign object. Also seek immediate medical care (call 911) for serious symptoms, such as anxiety with an effort to breathe; bluish skin, lips, or mucous membranes; high pitched sounds during inhalation; difficulty speaking due to lack of breath; drooling; lethargy; pale or blue lips; rapid heart rate; or severe difficulty breathing (when the child seems to struggle to take a breath). Seek prompt medical care if your child develops a fever, wheeze, or barking cough after several days of nasal stuffiness, or if your child is being treated for croup, but manageable symptoms recur or are persistent.