Philadelphia’s 1793 yellow fever outbreak was a defining moment in the history of the early American republic. Long-simmering tensions came to a boil as the epidemic claimed thousands of lives and spurred panic throughout the United States. Of the 40,000 people who lived in Philadelphia, yellow fever took the lives of 5,000 and caused another 20,000 to flee the city. Thousands of those who remained were left impoverished in an eerily silent city that would take years to recover. While the fever had a devastating impact on Philadelphia, it became a national crisis because it exposed deep-rooted conflicts in American society.

Despite the adoption of the Constitution in 1788, the young United States was a nation divided along geographic, economic, social, and cultural lines. In the arena of national politics, these tensions emerged most clearly in the clash between Federalists and Jeffersonian Republicans. Many Federalists, most prominently Alexander Hamilton, viewed cities as essential to the long-term survival and prosperity of the United States. Jeffersonians, by contrast, believed that the growth of cities would ultimately prove harmful to the nation by corrupting the moral character and civic virtue that they viewed as essential to a free society. The fever crisis forced Americans to confront these difficult questions about what kind of people they were and what kind of nation they would become.

Several factors magnified the significance of the 1793 epidemic beyond its local effects. First, Philadelphia’s commercial links connected the city to communities throughout the United States, raising fears that the disease might spread beyond the city. Second, Philadelphia was the nation’s capital. In the 1790 Residence Act, Federalists and Republicans agreed to establish a permanent capital in a new federal district on the Potomac River. Philadelphia would serve as the temporary capital until the site was ready; however, the near-collapse of government institutions during the crisis cast doubt on whether the government would survive long enough to make the move. Third, many suspected that the disease entered the country from the rebelling French colony of Saint Domingue (present-day Haiti), where a 1791 slave uprising had triggered a major flow of refugees into the United States. The fever crisis therefore raised important questions about the nation’s relationship with the broader world.

When the fever struck, it immediately sparked a medical debate, both in the city and throughout the United States. Many doctors, including the city’s College of Physicians, believed that the disease was an imported contagion. Benjamin Rush, perhaps the country’s most famous physician, led the opposing camp, arguing that the fever was produced by local environmental conditions. Both factions had grasped a part of the problem, but without germ theory doctors could neither explain the cause of the disease nor arrest its progress once a patient was infected. In truth, yellow fever is a blood-borne Flavivirus transmitted when an Aedes aegypti mosquito bites a healthy individual after feeding on a host carrying the disease. Symptoms usually begin with aches and nausea, but if the virus spreads to the liver, the victim might experience jaundice, hemorrhaging, coma, organ failure, and death. Indigenous to Africa, the virus had followed the slave trade to the Western Hemisphere, where it took root in the tropical Caribbean. North American ports had occasionally suffered the disease during the colonial era, but 1793 was the first outbreak in almost three decades.

The 1793 outbreak ended only after falling temperatures made the city too cold for the imported tropical mosquitoes to continue breeding. But that summer’s ordeal was only the first in a long series of outbreaks that would devastate American cities for decades to come. Still, the 1793 epidemic set the terms for how Americans attempted to control the scourge.