On February 13, 1898, the USS Maine exploded in a Havana harbor, killing 268 U.S. seamen. Blaming the country that controlled Cuba, the United States on April 25 declared war against Spain. U.S. troops then prepared to descend on an island where tropical diseases would prove to be their greatest enemy.

Warning Signs
Yellow fever was first reported in Cuba in 1649, when one-third of Havana residents died from the disease. From 1856 to 1879, the disease struck the city nearly every month. Foreign occupiers were particularly susceptible: an estimated 16,000 Spanish troops died from yellow fever between 1895 and 1898. At the onset of war with the United States, illness had decimated the Spanish fighting force, with 55,000 troops out of an army of 230,000 healthy enough to fight. U.S. officials were aware of the dangers from disease. Army Major Walter Reed, a physician who would later head the U.S. Army Yellow Fever Board in Cuba, offered advice to a friend who expected to be deployed there. Surmising that the germ for yellow fever was inhaled, Reed wrote that a "plug of cotton in the nostrils would be advisable."

Invasion
Despite knowing that yellow fever was most likely to strike in the summer rainy season, the U.S. invaded Cuba on June 22, when the Fifth Army Corps landed at Daiquiri. U.S. forces vanquished Spanish troops on the road to Santiago, but Lieutenant Colonel Theodore Roosevelt of the Rough Rider unit foresaw the dangerous conditions in a letter to Secretary of War Russell Alger: "If we are kept here it will in all human possibility mean an appalling disaster, for the surgeons here estimate that over half the army, if kept here during the sickly season, will die." But Alger ordered the troops to hold their ground in Santiago until Spanish forces surrendered. They did on July 17, but the damage had already been done.

Battle Against Disease
Fewer than 400 American soldiers were killed in combat during the war. But more than 2,000 contracted yellow fever during the campaign. Disease, said General William Shafter, was a "thousand times harder to stand up against than the missiles of the enemy." On July 6 in the town of Siboney, the first case of yellow fever among U.S. troops occurred. For the next several weeks, more troops were struck by malaria and dysentery. Yellow fever began to spread, which officers and doctors blamed on infected buildings in Siboney. In response, General Nelson Miles ordered Siboney evacuated and burned on July 11. An infantry unit of black soldiers, thought to be immune from yellow fever, were brought in to tend to the afflicted. More than a third of their regiment died from yellow fever or malaria.

Infection in Camps
Meanwhile, troops stationed away from the fighting also faced danger. When the Spanish-American War was declared, thousands of U.S. volunteers entered training camps in the southeastern United States. Though the surgeon general advised soldiers to maintain sanitary conditions, the camps were filthy, leading to a deadly typhoid outbreak. Of 171,000 personnel, 20,700 contracted the disease and more than 1,500 died. The U.S. Army Typhoid Board, lead by Major Walter Reed, was established to study the disaster in August 1898. The board subsequently visited the training camps and outlined sanitary measures for military commanders to follow to protect the troops.

The Occupation
Approximately 50,000 U.S. troops were stationed in Cuba at the conclusion of the war. U.S. officials focused on preventing future outbreaks of yellow fever and other diseases, which were all caused, according to Major William Crawford Gorgas, the chief sanitary officer in Havana, "by filth, dirt, and general unsanitary conditions." Cleanup measures helped control diseases spread by unsanitary conditions such as typhoid and dysentery. But by July 1899, yellow fever returned to Cuba, though to a lesser degree than the previous year. Gorgas later wrote: "The health authorities were at their wit's end. We evidently could not get rid of Havana as focus of infection by any method." In 1900, the U.S. Army established a special board of scientists to investigate the problem. In the end this board would prove, to the surprise and disbelief of many, that a common domestic mosquito spread the disease.