Croup

This figure shows the appearance of the vocal cords (larynx) and windpipe (trachea) in a child with croup. The viruses that cause croup initially infect the nose and pharynx and then spread along the upper respiratory tract to the larynx and trachea. As the infection progresses, the trachea and larynx become swollen, which narrows the available space for air to enter the lungs. The characteristic "barking cough" of croup is caused by the narrow airway.