Respiratory syncytial virus (RSV) is a highly contagious virus that causes an infection with symptoms similar to those of a moderate to severe cold. RSV infection usually resolves on its own and does not cause major health concerns, but it can become a problem when it is severe or leads to pneumonia, bronchiolitis, or other complications.

Babies (especially those born prematurely or with heart or lung problems), people with immune system problems, and adults age 65 and older have an increased risk of developing complications from RSV infection.

Although reinfection with RSV is common throughout life, the first infection usually causes the most severe symptoms. Almost all children become infected with RSV by the age of 2 years.