The purpose of this study is to determine whether immune globulin can limit the amount of yellow fever vaccine virus present in the blood after vaccination without compromising the immunity associated with the yellow fever vaccine. The study will enroll 80 participants in two groups of 40 each. The first group will receive the yellow fever vaccine with salt-water placebo. The second group will receive yellow fever vaccine with immune globulin. The amount of vaccine virus and immune response in both groups will be compared. Yellow fever vaccine has been used to protect humans against Yellow Fever Vaccine disease since the 1930s.