During the hot, humid summer of 1793, thousands of Philadelphians got horribly sick, suffering from fevers and chills, jaundiced skin, stomach pains and vomit tinged black with blood.

By the end of August, as more and more people began dying from this mysterious affliction, wealthier residents of the nation’s capital were fleeing in droves. The city’s free black community, meanwhile, largely stayed behind and many were enlisted to help care for the sick.

“It is called a yellow fever, but is like nothing known or read of by the Physicians,” wrote Secretary of State Thomas Jefferson in September 1793.

At the time, no one knew what caused yellow fever, or how it spread. Some thought it had been brought to Philadelphia by a ship bearing French refugees from a slave rebellion in Santo Domingo (now Haiti). Others—including the city’s leading physician, Dr. Benjamin Rush—believed it originated in the poor sanitary conditions and contaminated air of the city itself.

However the disease had arrived, Philadelphians in 1793 desperately sought to avoid getting it. They began keeping their distance from each other and avoided shaking hands. They covered their faces with handkerchiefs dipped in vinegar or smoked tobacco, which they thought would prevent them from breathing in contaminated air.

Those who had the means to leave the city quickly did so, including Jefferson himself. President George Washington, who returned to his beloved Mount Vernon estate, blamed his exit on the concerns of his wife, Martha.

Alexander Hamilton contracted yellow fever early in the epidemic, and he and his family left the city for their summer home a few miles away. Hamilton’s wife, Eliza, soon fell ill as well, and their children were evacuated to Eliza’s parents home in Albany, New York. They both recovered under the care of Dr. Edward Stevens, a boyhood friend of Hamilton’s from St. Croix whom he met again in Philadelphia.

WATCH: 'Hamilton: Building America' on HISTORY Vault

Among the mass exodus of some 20,000 Philadelphians—nearly half the city’s total population at the time—during the yellow fever epidemic were many of the city’s doctors, who were terrified of getting ill themselves. But Rush, the country’s most prominent medical professional and a signer of the Declaration of Independence, stayed behind, working tirelessly to treat rich and poor patients alike. Rush lost his sister to the disease and even fell ill himself, although he recovered.

Despite all his efforts, Rush had just a flawed understanding of yellow fever as anyone else at the time. His undeniably harsh treatments—including bloodletting, “Mercurial Sweating Powder,” and forced vomiting—did not curb the spread of the disease, and critics argued it only increased his patients’ suffering. These critics included Hamilton, who took up his pen to spread the word of the gentler methods prescribed by his own doctor, which involved taking cold baths, drinking Madeira wine and hot brandy and ingesting large amounts of quinine (aka “Peruvian bark”), according to biographer Ron Chernow.

Stevens’ homeopathic approach proved little more effective than Rush’s more traditional methods, however, and yellow fever continued to spread. By the time it subsided in November 1793, the disease had killed 5,000 people, or about one-tenth of Philadelphia’s population at the time, and infected hundreds of thousands of others. Despite extensive research on the disease in the decades that followed the epidemic, it would take more than a century—and a savage outbreak among troops fighting the Spanish-American War—before Dr. Walter Reed proved in 1900 that mosquitoes carried yellow fever.

"Parents desert their children as soon as they are infected, and in every room you enter you see no person but a solitary black man or woman near the sick,” Rush wrote to his wife, Julia, who was in Princeton, New Jersey, with the couple’s children, during the 1793 epidemic. “Many people thrust their parents into the street as soon as they complain of a headache.”

As his letter indicates, Rush enlisted members of Philadelphia’s free African-American community to treat many of the fever’s victims as well as do much of the essential labor necessary to keep the city going during the epidemic. He and other white physicians initially (and wrongly) believed African Americans were immune to yellow fever due to supposed biological differences based on race.

Rush was an ardent abolitionist, and had been supportive of the efforts of the city’s black community to form their own churches in protest over the segregation of white-led ones. Led by Richard Allen, the co-founder of the African Methodist Episcopal Church, and his fellow minister Absalom Jones, black volunteers provided crucial labor during Philadelphia’s yellow fever epidemic.

When the publisher Mathew Carey, who served on the city’s health committee, issued his account of the epidemic beginning in October 1793, he accused members of Philadelphia’s free black community of profiting off the epidemic, even stealing from the houses of fever victims. In response, Allen and Jones published their own pamphlet in early 1794 refuting these accusations in detail. By including eyewitness testimony of the work black Philadelphians did to treat patients, along with detailed documentation of payments and expenses, the two ministers forced Carey to revise his chronicle of the epidemic in later editions.

Allen and Jones’ work was the first copyrighted pamphlet written by black authors in the nation’s history. Titled A Narrative of the Proceedings of the Black People, During the Late Awful Calamity in Philadelphia, in the Year 1793, it documented the racism and poor treatment that free African Americans experienced, even as they played a crucial role in combating the most serious epidemic of disease in the history of the still-young nation.