Ted Stewart, age 15, of Reno, Nev., for his question:

WHAT CAUSES CROUP?

Croup is a disease of the respiratory tract that is characterized by a harsh, barking cough and great difficulty in breathing. Croup that happens often may be due to an allergy and usually each attack clears up by itself.

Croup with sudden onset may be due to infection of the epiglottis by bacteria. This condition, called epiglottitis, can result in complete blockage of the airways and must be treated by a doctor.

Therapy for croup sometimes involves the use of an antibiotic to fight the infection and passage of a tube by the doctor into the airway to maintain breathing.

Epiglottitis happens most often in children between the ages of 2 and 6. Croup may also be caused by viral infection of the lower part of the trachea. This illness is often preceded by signs of respiratory infection and most often happens between the ages of 6 months and 3 years. A croup like cough may also signal the presence of a foreign body in the airway.