This is a Phase 1, double-blind, placebo controlled study enrolling 42 healthy adult subjects (18-45yrs) from 1 site. Subjects will be randomized in a 2:3:2 ratio to receive MEDI-557 or placebo. Subjects will receive 1 intravenous dose on Study Day 1. Subjects will be followed for safety from the time of Informed Consent through 360 days post dose.