Most infants are exposed to the respiratory syncytial virus
(RSV) by their second birthday. 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. Viruses are biological
agents made of genetic materials and proteins that are able to
infect a host (a living organism) and replicate. RSV can enter
the body through the eyes, nose, or