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 bron-chiolitis 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.

The clinical disease in young infants may be the result of an antigen-antibody reaction that occurs when the infecting virus meets antibody transmitted from the mother. For this reason respiratory syncytial vaccines that stimulate production of antibodies in the serum, but not in the nasal secretions, may do more harm than good. Animal virus