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.

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.

Inclusion Criteria:

• 18-45 years
• written informed consent obtained from subject prior to performing any protocol related procedures
• healthy by medical history and physical exam
• females of child-bearing potential must use 2 effective methods of birth control for 14 days prior to 1st dose and through 1 year after administration of study drug
• nonsterilized, sexually active males with female partner of child-bearing potential must use 2 effective methods of birth control from Day 1 through Day 361
• weight </= 110kg with a body mass index of <32kg/m2
• ability to complete a follow-up period of approximately 360 days

Exclusion Criteria:

• inability to complete a follow-up period of 360 days
• any condition in the opinion of investigator that would interfere with evaluation of IP or interpretation of subject safety or study results
• concurrent enrollment in another clinical study
• employees of the site or other individuals involved with the conduct of the study or immediate family members of such individuals
• receipt of immunoglobulin or blood products within 60 days prior to randomziation
• receipt of any investigational drug therapy within 6 months prior to IP dosing
• clinically abnormal ECG at screening
• blood donation in excess of 400mL, wihtin 6 months prior to randomization
• previous receipt fo biologics
• history of immunodeficiency
• history of allergic disease or reactions likely to be exacerbated by any component of the IP
• previous medical history or evidence of interurrent illness that may compromise the safety of the subject
• positive lab test for Hep A, B, C or HIV
• pregnancy or nursing mother
• history of alcohol or drug abuse within past 2 years
• positive urine Class A drug screen
• acute illness within 7 days prior to randomization
• fever >/= 99.5F witin 7 days prior to randomization
• any drug therapy within 7 days prior to randomization
• systolic BP >150mmHG and/or diastolic BP>90mmHg
• receipt of vaccine within 14 days prior to randomization
• abnormal study labs (hem/wbc/platelet/BUN - see protocol for specific information)