The exceptions to this are yellow fever and dengue viruses , which still require mosquito vectors , but are well-enough adapted to humans as to not necessarily depend upon animal hosts ( although both continue to have important animal transmission routes as well ) .
The exceptions to this are yellow fever and dengue viruses , which still require mosquito vectors , but are well-enough adapted to humans as to not necessarily depend upon animal hosts ( although both continue have important animal transmission routes as well ) .