Respiratory syncytial virus (RSV) is a very common virus that leads to mild, cold-like symptoms in adults and older healthy children. It can be more serious in young babies, especially those in certain high-risk groups. RSV is the most common germ causing lung and airway infections in infants and young children. Most infants have had this infection by age 2. Outbreaks of RSV infections most often begin in the fall and run into the spring. The infection can occur in people of all ages. The virus spreads through tiny droplets that go into the air when a sick person blows their nose, coughs, or sneezes.