Human respiratory syncytial virus (RSV) is a common cause of upper respiratory tract illnesses as well as more severe lower respiratory illnesses, including bronchiolitis and pneumonia. RSV affects almost all children within the first 2 years of life. This study will evaluate the safety and immune response to the RSV MEDI ΔM2-2 vaccine among adults, RSV-seropositive children, and RSV-seronegative infants and children.