Most infants are exposed to the respiratory syncytial virus (RSV) by their second birthday, and almost all children are infected by the age of 4. RSV can cause a serious lung infection in infants and younger children and is more common in premature babies and infants or younger children with health problems, such as heart or lung disease.
Seniors and adults with heart or lung problems may also experience a severe infection. People with a weakened immune system are also at risk. RSV does not cause such a serious infection for healthy children and adults, who may experience symptoms similar to the common cold.
Even though people, including infants, develop antibodies (immune defenses) against the virus after an infection, it appears the virus is able to reinfect individuals throughout their lifetime. In cases of reinfection, symptoms are usually less severe than the first infection.