Safety assessments in vaccinated subjects include adverse event reporting and clinical laboratory evaluations. To ensure thorough reporting of adverse events during the time interval in which adverse events associated with yellow fever vaccination might be expected to occur, subjects will be required to complete a study diary for Days 0-28. The study diary will record both local reactions (i.e., pain, redness, and swelling) and systemic reactions (i.e., feeling ill [malaise], headache, muscle aches, fever, chills, rash, tiredness, and other general symptoms). These are the principal symptoms and side effects known to be associated with yellow fever 17D vaccine. At each visit, study personnel will perform clinical evaluations and at days 7, 14 and 30 the study staff will review the diary card and follow up on any adverse events that occurred since vaccination.