Respiratory syncytial virus (RSV) is a virus that causes infections of the lungs and respiratory tract. It's so common that most children have been infected with the virus by age 2. Respiratory syncytial (sin-SISH-ul) virus can also infect adults.
In adults and older, healthy children, the symptoms of respiratory syncytial virus are mild and typically mimic the common cold. Self-care measures are usually all that's needed to relieve any discomfort.
Infection with respiratory syncytial virus can be severe in some cases, especially in premature babies and infants with underlying health conditions. RSV can also become serious in older adults, adults with heart and lung diseases, or anyone with a very weak immune system (immunocompromised).