Yellow fever was an ancient scourge of 18th and 19th century battlefields, causing more fatalities than bullets, cannon, or swords. The dreadful disease, which was brought from Africa to the tropical Americas by the slave trade, struck military camps without warning, decimating entire armies.
Mosquito abatement programs pioneered by William Gorgas in Cuba and Panama led to immediate relief from yellow fever. Later, his discoveries played a major role in WWII, and still help with control of the disease today. We have a vaccine, developed by Max Theiler, but there is still no cure and outbreaks occur.
The loss of many lives to yellow fever during the 1898 Spanish American War prompted an investigation into how people caught the disease. Major Walter Reed is credited with proving, through a series of experiments with volunteers, that mosquitoes, not contaminated objects, are to blame.