The primary aim of this study is to determine the difference between antibody titers in the two study groups and study the relationship between endemic infections, IA, the FRCn, and CD4 and CD8 T cell subsets and the magnitude and durability of neutralizing antibody response to YFV in a cohort shown to have elevated IA, a damaged FRCn, and pan T cell depletion and a cohort that does not. This is a single arm, open-label, two cohort study of healthy adults in Kampala, Uganda and in Minnesota, USA. The cohort in Uganda will be 30 adults (15 men and 15 women) and the cohort in Minnesota will be 16 adults (8 men and 8 women). Everyone will be screened to ensure there are no contraindications to receiving YFV (e.g., immunosuppression) or the planned procedures. The inclusion and exclusion criteria are discussed in detail in the protocol that is included in the appendix. Participants will have an inguinal LN biopsy, leukapheresis, and GALT biopsies prior to YFV and again 3 weeks after the vaccine administration. The vaccine will be given in the contralateral thigh from the first LN biopsy so that the second biopsy will be from a draining LN. PBMC and plasma will be collected at regular intervals over the 18-month follow-up period and leukapheresis will be done again at the month 18 visit.