A SOLDIER LIES in a tent hospital in Siboney, Cuba, in July 1898, a victim of yellow fever. That month, senior U.S. Army officers fresh from victories at San Juan Hill and Santiago proposed immediate evacuation: “The army is disabled by malarial fever to such an extent. … that it is in a condition to be practically entirely destroyed by the epidemic of yellow fever sure to come. …” Spain surrendered before President McKinley had to weigh the risks of an epidemic against the humiliation of withdrawal. Even so, losses were severe: in the combined theaters of the war, fourteen times as many men died of tropical diseases as from enemy action.

Pressure to conquer malaria and yellow fever did not end with the armistice: to maintain control of Cuba, the Army planned to station ten regiments there. Doctors had quinine to treat malaria, but no drug had proved useful against yellow fever. In an attempt to avert an epidemic, the new recruits were drawn from Southern states already hit by yellow fever—men thought to be immune. When even these men started to fall ill, despite all efforts at sanitation and quarantine, the surgeon general appointed a board to investigate the disease.

Led by Maj. Walter Reed, the board tested a theory, put forward by a Cuban physician named Carlos Finlay, that mosquitoes spread yellow fever. After a series of experiments in which Army volunteers were deliberately infected, Reed proved conclusively that the Aedes aegypti mosquito was the carrier. Maj. William Gorgas then began a campaign to eradicate the insect, and within a year there were no yellow fever cases in Havana for the first time in over a hundred years. Reed’s work was immediately put to use by Gorgas in a peacetime project: cleaning up the Panama Canal Zone.