Epidemics have been pivotal in the history of the world as exemplified by a yellow fever epidemic in the Caribbean that clearly altered New World geopolitics. By the end of the 18th century, yellow fever—then an “emerging disease”—was widespread throughout the Caribbean and particularly lethal in Saint-Domingue (present day Haiti). From 1793 to 1798, case fatality rates among British troops in the West Indies (including Saint-Domingue) were as high as 70%. A worse fate befell newly arrived French armed forces in 1802, ostensibly sent by Napoleon to suppress a rebellion and to reestablish slavery. Historians have disagreed on why Napoleon initially dispatched nearly 30 000 soldiers and sailors to the island. Evidence suggests the troops were actually an expeditionary force with intensions to invade North America through New Orleans and to establish a major holding in the Mississippi valley. However, lacking knowledge of basic prevention and control measures, mortality from the disease left only a small and shattered fraction of his troops alive, thwarting his secret ambition to colonize and hold French-held lands, which later became better known as the Louisiana Purchase. If an event of the magnitude of France's experience were to occur in the 21st century, it might also have profound unanticipated consequences.