Respiratory Syncytial Virus (RSV) is a human restricted pathogen and is the single most important cause of severe respiratory illness in infants and young children, a major cause of infantile bronchiolitis and is the most frequent cause of hospitalization of infants and young children in industrialized countries. Severe RSV infection early in life is associated with an increased risk of subsequent recurrent wheezing and asthma. There are few population-based estimates of the incidence of RSV disease from developing countries, but the existing data clearly indicates that the virus accounts for a high proportion of Acute Respiratory Infections (ARI) in children. Studies in Brazil, Colombia and Thailand suggest that RSV causes 20-30% of ARI cases in children from 1-4 years of age, a proportion similar to that in industrialized countries, and WHO has estimated the global RSV disease burden at 64 million cases and 160 000 deaths every year. RSV also causes severe disease in elderly and immune-compromised adults, and the burden of RSV disease in the elderly is comparable to that of seasonal influenza. The economic impact of RSV-related disease in adults estimated to be greater than that of influenza in relation to numbers of days lost from work.