The primary objective of this study is to describe the safety and tolerability of 3 doses of MEDI-560 at 10^5 TCID50 when administered to children 6 to < 12 months of age who are HPIV3 (human parainfluenza virus type 3) seronegative at baseline and to infants 1 to < 3 months of age regardless of baseline serostatus.