The risk for infection with a virus that causes colds, flu, and other respiratory illnesses increases with each child in a family, according to a study published online ahead of print August 5 in Clinical Infectious Diseases. However, only half of these infections may lead to illness.

Krow Ampofo, MB, ChB, and Carrie Byington, MD, and colleagues monitored 26 households and a total of 108 individuals for viral infection and symptoms of cold and flu for one year. People living without children were infected with viruses for an average of three to four weeks during the year. In households with one child, the average was 18 weeks, and for those with six children, it was up to 45 weeks out of the year. Participants who were infected with a rhinovirus had symptoms half of the time, but those infected with influenza and parainfluenza viruses exhibited symptoms most of the time.

Children aged younger than 5 years had at least one virus detected in their nasal mucus for 50% of the year, which was twice as often as older children and adults. In addition, when infected, they were 1.5 times more likely to be symptomatic.

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