Epidemiology

Croup syndromes occur worldwide. Most illnesses are due to the common croup viruses: parainfluenza types 1 and 2 and influenza viruses. Outbreaks occur every year during the cold-weather months; in the tropics croup is more common during the rainy season. The highest attack rate occurs in children 7 to 36 months of age; few cases occur in children after the sixth birthday. During the second year of life, about 5 percent of children experience an episode of croup. Croup is more common in boys than in girls and also tends to be more severe in boys.

The viruses that cause croup are present in the nasal secretions of adults and children with colds and other upper and lower respiratory tract illnesses. Virus is transmitted from infected persons by sneezing, nose blowing, and the general contamination of external surfaces (including the hands) with nasal secretions. A susceptible child can become infected either by inhaling virus in droplet nuclei (small particles) or by a direct nasal hit of virus-containing large droplets from a sneeze or nose blowing. Infection can also occur indirectly as a result of contamination of the fingers of the recipient. It is important to emphasize that parainfluenza virus infections in adults are manifested by colds; older persons with relatively trivial illnesses may be the source of severe croup in young children.

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