Respiratory Syncytial (sin-SISH-uhl) Virus, or RSV, is a common respiratory virus that usually causes mild, cold-like symptoms. Most people recover in a week or two, but RSV can be serious, especially for infants and older adults. RSV is the most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia (infection of the lungs) in children younger than 1 year of age in the United States. There are an estimated 177,000 annual hospitalizations and 14,000 deaths related to RSV in adults aged 65+.

RSV in immunocompromised persons frequently results in lower respiratory tract infection.