This is the first map that animates historic yellow fever outbreaks in the U.S. How and why this disease moved during the 1800s is poorly understood. Before 1820, yellow fever was largely confined to the Eastern seaboard. After this point, it kills largely in the South. It also moves up the Mississippi River and inland from the Texan Gulf. I argue that while changing demographics and new technologies that transported the mosquito vector facilitated the disease, we should also consider naturally occurring changes of the distribution of mosquitoes to be an important factor.