Virtually every baby at some point or other 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 runny nose, cough or fever – and suffer no long-term health consequences.

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