This is a randomized, double-blind, placebo-controlled, multidose Phase 1/2a multicenter study designed to evaluate the safety, tolerability, viral shedding, immunogenicity, and genotypic and phenotypic stability of MEDI-560 in infants 1 to < 12 months of age. Three doses of MEDI-560 at a dosage level of 10^5 TCID50 were administered 0, 2, and 4 months after enrollment to a 30-participant cohort of 6 to < 12 month-old HPIV3 seronegative children randomized 2:1 to MEDI-560 vs placebo. A second 160-participant cohort of 1 to < 3 month-old infants not screened for baseline serostatus was planned but was not opened to enrollment for reasons other than safety. Participants were followed for safety through 180 days post last dose. Nasal wash specimens were collected at screening and Days 7, 12, and 28 following each dose and during unscheduled illness visits to assess vaccine virus shedding and genotypic and phenotypic stability of any shed vaccine virus. Blood was collected at screening to determine eligibility and prior to Dose 1 for baseline serostatus. Blood for assessment of antibodies to HPIV3 was collected approximately 7 to 12 days after Dose 1 and Dose 3 and 1 month after each dose for antibodies to PIV3.