Respiratory syncytial virus (RSV) is a very contagious infection of the lungs and breathing passages that affects small children. Almost all children contract RSV by the time they turn two, most of the time, symptoms are limited to those of the common cold. In premature babies and those with lung, heart or immune system conditions, RSV can be serious and require hospitalization until symptoms get better. There are medications that can help prevent RSV infection in high-risk children.