Yellow fever virus (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. In this review we consider evolutionary and ecological 
factors underlying YFV emergence, maintenance and spread, geographic 
distribution and patterns of epizootic/epidemic activity.