This study is a multi-center, double-blind, placebo-controlled, Phase I study to evaluate the safety, reactogenicity, tolerability, and immunogenicity of MVA-BN-YF in Flavivirus-naïve healthy male and non-pregnant female adult subjects. There are six dose groups in this study. Subjects who have never received a licensed or investigational smallpox vaccine will be randomized to Groups 1-5 and vaccine administration and follow-up will be conducted in a double-blinded fashion. Subjects who have previously received two, 1 x 108 TCID50 doses of MVA-BN between 19 and 45 days apart by SC or IM routes will be enrolled in Group 6 and will be dosed open-label. Since this is a first in human, phase I study, a sentinel cohort will be utilized. The first two subjects (1st sentinel group) one at each clinical site will be randomized to Group 2 or 3 and vaccinated with MVA-BN-YF with or without ISA 720. Subjects and study personnel will be blinded as to whether ISA 720 was administered.