Human respiratory syncytial virus (RSV) is the most common viral cause of serious acute lower respiratory illness (LRI) in infants and children under 5 years of age worldwide. This study will evaluate the safety, infectivity, and immunogenicity of a single dose of RSV LID cp ΔM2-2, a recombinant live-attenuated RSV vaccine, in RSV-seronegative infants and children 6 to 24 months of age.