Respiratory syncytial virus (RSV) infects almost all children at least once before they are 2 years of age. Most of the time, this virus only causes minor cold-like symptoms. However, for some babies infection can be more dangerous.

For certain infants who are born extremely preterm (before 29 weeks of pregnancy) or who are born with severe heart or lung disease, any viral infection can be more serious. Very preterm infants often have underdeveloped lungs and may have difficulty fighting a viral infection once they become infected.

Each year, about 100,000 children are hospitalized in the United States with RSV infection, and approximately 100 of these children will die. In the first 2 years of life, RSV is the leading cause of pneumonia and bronchiolitis (a swelling of the small airways), and it may be associated with wheezing.

Infants born prematurely and term infants younger than 6 months are at...

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