YFV) is historically one of the most important viruses to affect human populations. Despite the existence of highly effective vaccines for over 70 years, yellow fever remains a significant and re-emerging cause of morbidity and mortality in endemic and high-risk regions of South America and Africa. The virus may be maintained in sylvatic enzootic/epizootic, transitional and urban epidemic transmission cycles with geographic variation in terms of levels of genetic diversity, the nature of transmission cycles and patterns of outbreak activity.