Respiratory syncytial virus (RSV) causes respiratory tract infections and occurs seasonally. It is the primary etiological agent associated with the majority of serious lower respiratory tract infections and hospital visits during infancy and childhood. There is currently no effective therapy for the treatment of this debilitating disease. 

In the United States, 60% of infants are infected during their first exposure to the RSV season, and close to 100% of children will have been infected with the virus by the age of 2. Of those infected with RSV, 2–3% will go on to develop bronchiolitis, requiring hospitalization. On occasion, an infant can become symptomatically infected more than once, even within one RSV season. Increasingly, severe RSV infections have been found among elderly patients.

RSV is a negative-sense, single-stranded RNA virus of the family Paramyxoviridae, that includes many common respiratory viruses and the viruses that cause measles and mumps. RSV is a member of the paramyxovirus subfamily Pneumovirinae.