This is a Phase 1/2a, randomized, double-blind, placebo-controlled, dose-escalation, multicenter study to evaluate the safety and tolerability of multiple doses of MEDI-534 at 10^5 or 10^6 median tissue culture infectious dose (TCID50) in RSV and PIV3 seronegative children 6 to <24 months of age and at dosages of 10^4, 10^5 or 10^6 TCID50 in unscreened infants 2 months of age.