Respiratory syncytial virus: A virus that causes mild respiratory infections, colds, and coughs in adults and can produce severe respiratory problems, including bronchitis and pneumonia, in young children and anyone with compromised immune, cardiac, or pulmonary systems. Abbreviated RSV. RSV is spread via respiratory secretions and is highly contagious. Infections usually occur during annual community outbreaks, often lasting 4 to 6 months, during the late fall, winter, or early spring. RSV typically features fever, prominent nasal secretions, and congestion coupled with wheezing for 1 to 2 weeks. Having immunity against RSV requires having a continuous, solid level of antibodies against the virus. There is particular concern about RSV occurring in premature babies, because their immune systems lack maturity and antibodies. There is no RSV vaccine.