This is a Phase 2, randomized, double-blind, placebo-controlled, multicenter study to determine the effect of a single 30 mg/kg IM dose of motavizumab on viral load and motavizumab levels in the upper respiratory tract of children who present with RSV illness but who do not require hospitalization. Using 1:1 randomization, 30 mg/kg motavizumab or placebo will be administered as soon as possible after a child's diagnosis of RSV and his/her eligibility for the study has been confirmed.