Croup is usually caused by a virus infection, most often a parainfluenza virus.

Your child may contract a virus by breathing infected respiratory droplets coughed or sneezed into the air. Virus particles in these droplets may also survive on toys and other surfaces. If your child touches a contaminated surface and then touches his or her eyes, nose or mouth, an infection may follow.

The infection usually starts in the nasal membranes and then spreads to the vocal cords (larynx) and windpipe (trachea). Only a small percentage of exposed children actually develop croup.

Jan. 30, 2013