Two different species of flaviviruses, dengue virus (DENV) and yellow fever 
virus (YFV), that originated in sylvatic cycles maintained in non-human primates 
and forest-dwelling mosquitoes have emerged repeatedly into sustained 
human-to-human transmission by Aedes aegypti mosquitoes. Sylvatic cycles of both 
viruses remain active, and where the two viruses overlap in West Africa they 
utilize similar suites of monkeys and Aedes mosquitoes. These extensive 
similarities render the differences in the biogeography and epidemiology of the 
two viruses all the more striking. First, the sylvatic cycle of YFV originated 
in Africa and was introduced into the New World, probably as a result of the 
slave trade, but is absent in Asia; in contrast, sylvatic DENV likely originated 
in Asia and has spread to Africa but not to the New World. Second, while 
sylvatic YFV can emerge into extensive urban outbreaks in humans, these 
invariably die out, whereas four different types of DENV have established human 
transmission cycles that are ecologically and evolutionarily distinct from their 
sylvatic ancestors. Finally, transmission of YFV among humans has been 
documented only in Africa and the Americas, whereas DENV is transmitted among 
humans across most of the range of competent Aedes vectors, which in the last 
decade has included every continent save Antarctica. This review summarizes 
current understanding of sylvatic transmission cycles of YFV and DENV, considers 
possible explanations for their disjunct distributions, and speculates on the 
potential consequences of future establishment of a sylvatic cycle of DENV in 
the Americas.