Respiratory syncytial virus is a common infection of the respiratory tract that strikes virtually all children by the age of 2. More commonly known as RSV, the infection is highly contagious. It is frequently associated with epidemics that can last up to five months and that typically occur in late fall, winter or early spring. 

RSV can enter the body through the eyes, nose or mouth. Children over age 3 and most adults who contract RSV usually experience symptoms that are no worse than those of a common cold. However, RSV in children younger than 3 can sometimes cause lower respiratory tract illnesses such as bronchiolitis (commonly called a "chest cold"), tracheobronchitis (croup) and pneumonia. In rare but severe cases, RSV can lead to respiratory failure.

RSV is most likely to cause more serious symptoms in babies born prematurely, older adults, and those with heart, lung or immune system problems, regardless of age.

There appears to be a link between bouts of RSV and increased incidences of asthma symptoms. However, scientists have not concluded that RSV actually causes asthma. Rather, it is thought that RSV causes swelling and narrowing of the lower airway that can trigger asthma symptoms. However, another study on RSV has also indicated that it could contribute to the development of asthma. Research is ongoing.