Basic epidemiology and immunopathology of RSV in children.

Respiratory syncytial virus (RSV) is the most important cause of lower respiratory tract infection in infants and young children. Around 20 000 RSV-infected infants require hospitalization in the UK during each yearly epidemic, which is about 3% of the birth cohort. Most children are infected by 2 years of age. Risk factors for severe disease include young infants, prematurity, chronic lung and cardiac conditions or immunodeficiency. Humoral immunity is incomplete and short-lived, yet reinfections cause less severe disease. RSV infects infants despite the presence of specific neutralizing antibodies. RSV infection can be linked to the development of individual wheezing episodes. A competent cellular immune system is necessary to reduce disease severity. RSV infection provokes an RSV-specific T-lymphocyte response with the release of cytokines. There is a delicate balance between the protective and disease-enhancing effects of the host's immune response to RSV infection.