In the late eighteenth century, yellow fever was widespread in the Caribbean; case-fatality rates among British troops there were as high as 70 percent. The fate of French troops sent to Saint-Domingue to suppress a slave rebellion was even worse. “Evidence suggests the troops were actually an expeditionary force with intentions to invade North America through New Orleans and to establish a major holding in the Mississippi valley,” wrote the authors of a 2013 scholarly paper. Mortality from the disease thwarted Napoleon’s “secret ambition to colonize and hold French-held lands, which later became better known as the Louisiana Purchase.”