A virus belonging to the Paramyxoviridae, genus Pneumovirus. This virus, although unrelated to any other known respiratory disease agent and differing from the parainfluenza viruses in a number of important characteristics, has been associated with a large proportion of respiratory illnesses in very young children, particularly bronchiolitis and pneumonia. It appears to be one of the major causes of these serious illnesses of infants. It is the only respiratory virus that occurs with its greatest frequency in infants in their first 6 months of life. In older infants and children, a milder illness is produced. Respiratory syncytial virus infection in adult volunteers gave rise to a coldlike illness; adult infection took place readily, even though the adults had moderate to high levels of antibody.