This double-blind, randomized outpatient study is designed to determine if human immune globulin (IG) will limit the viremic response to 17D yellow fever vaccine without compromising immunogenicity. The yellow fever 17D vaccine YF-VAX® is manufactured in the United States. The IG, Gama STAN S/D®, is manufactured in the U.S. The study will be conducted in 80 healthy adults 18-40 years of age in the U.S. Participants will be randomized to receive YF-VAX® plus saline (40 subjects), or YF-VAX® plus Gama STAN S/D® (40 subjects). The dose of Gama STAN S/D® will be that used for the prevention of hepatitis A, i.e. 0.06 mL/kg. The lot of Gama STAN S/D® will have been tested and shown to contain yellow fever neutralizing antibodies at a log neutralizing index of >0.7. Saline will be given to blind the vaccine administrator and vaccinee as to their treatment group. A stratified randomization procedure will be used to ensure equal distribution of study medications to participants by gender. Safety and tolerability will be assessed by comparison of the incidence of adverse events across the two treatment groups.