Virtually every baby at some point gets respiratory syncytial virus (RSV), a respiratory bug that infects the lungs and breathing passages. It's the most common respiratory virus in infants and toddlers. Most kids with RSV have symptoms like that of a common cold – such as a runny nose, cough or fever – and suffer no long-term health consequences.

But in young infants – particularly children born prematurely or with certain health conditions – RSV can cause serious, even life-threatening complications. In the United States, up to 80,000 infants a year are hospitalized with severe RSV infections, making it the country's leading cause of infant hospitalization.