RSV is a respiratory virus that causes illness and trouble breathing. It is more common in winter and early spring months and can be which can be life-threatening in babies and the elderly people. RSVs It’s the most common cause of inflammation of the small airways in the lungs (bronchiolitis) and pneumonia in babies. Actually, most babies have been infected at least once by the time they are 2 years old. Babies can also be re-infected with the virus. Infection can happen again anytime throughout life.