A Phase 1, randomized, double-blind, placebo-controlled study evaluating the safety and tolerability of MEDI-557 administered IV to healthy adult subjects (18-45yrs). A maximum of 42 subjects from 1 site will be enrolled in a 2:3:2 ratio (12 placebo, 18 MEDI-557 low-dose, 12 MEDI-557 high-dose) to allow for greater data collection in the low-dose group. Subjects are evaluated for safety from time of Informed Consent through Study Day 360 post dose.