Respiratory Syncytial Virus Infection and Pathogenesis Respiratory Syncytial virus (RSV) is a major cause of serious lower respiratory tract disease in infants and young children. Hospitalized children with underlying heart and lung conditions are at the most risk for severe complications. No effective vaccine is available, and the current antiviral drug available has limited use. A nonhuman primate model of RSV infection and disease has been developed and is being utilized to test various RSV vaccine candidates and therapeutics.