The first of two agents on our list spread by the Aedes
mosquito, the yellow fever virus wasn't been much of a concern in the latter half of the twentieth century. Malaria control efforts in the 1950s successfully decimated the Aedes
population, and with it the occurrence of yellow fever. In the past few decades, however, the mosquito has returned and is ranging much further than previous generations. It's also making its way into urban environments, which it has done in the past—an outbreak nearly wiped Memphis off the map in 1878—but in recent memory, it has been confined to the tropical jungles.
The fever gets its name from the jaundice it can cause after a few days of infection. Later comes internal bleeding (it's a hemorrhagic fever like Ebola and Marburg) followed by bloody vomit with the consistency of coffee grounds. What is most worrying about its return to cities is that it achieves a higher mortality rate among dense, unexposed populations—up to 30 percent. Recent outbreaks in Paraguay and the Ivory Coast have health officials racing to vaccinate as quickly as possible. While an effective vaccine exists, there is no treatment and no cure.