Is This Really Important in Transmission of Yellow Fever?

A vertically transmitted infection means that female mosquitoes transmit the virus to her eggs. When those eggs hatch, larval mosquitoes are already infected with the yellow fever virus. And this is really important because that means that when they emerge as adults and are about to take their first blood meal, they can infect that first organism that they bite. Whereas with malaria and a lot of other mosquito-borne diseases, the mosquito has to get infected by taking a blood meal first. That would be just horizontal transmission.