HPIV type 3 (HPIV3) ranks second only to respiratory syncytial virus as the most common cause of bronchiolitis and pneumonia in infants less than 6 months of age. HPIV3 can cause severe disease in the first 2 years of life and is responsible for 11% of hospitalizations for respiratory diseases in children. This study will evaluate the safety and immunogenicity of a live, chimeric bovine/human, attenuated intranasal HPIV3 vaccine, rB/HPIV3. This vaccine combines modified human HPIV3 with a related, modified cow virus, bovine parainfluenza type 3 virus (BPIV3). Vaccinations will be given as nose drops to healthy adults, children seropositive for HPIV3, and infants and children seronegative for HPIV3.