During the 1793 Yellow Fever epidemic in Philadelphia, 5,000 or more people were listed in the official register of deaths between August 1 and November 9. The vast majority of them died of yellow fever, making the epidemic in the city of 50,000 people one of the most severe in United States history. By the end of September, 20,000 people had fled the city, including congressional and executive officials of the federal government. Most did not return until after the epidemic had abated in late November. The mortality rate peaked in October before frost finally killed the mosquitoes and brought an end to the outbreak. Doctors tried a variety of treatments but knew neither the origin of the fever nor that the disease was transmitted by mosquitoes (this information was not verified until the late 19th century).
The mayor and a committee of two dozen organized a fever hospital at Bush Hill and other crisis measures. The assistance of the Free African Society was requested by the city and readily agreed to by its members. Parties mistakenly assumed that people of African descent would have the same partial immunity to the new disease as many had to malaria, which was typically the most common source of fever epidemics during the summer months. Black nurses aided the sick, and the group's leaders hired additional men to take away corpses, which most people would not touch. But black people in the city died at the same rate as whites, about 240 altogether.
Some neighboring towns refused to let refugees in from Philadelphia, fearing that they were carrying the fever. Major port cities, including those in Baltimore and New York City had quarantines against refugees and goods from Philadelphia, although New York City sent financial aid to Philadelphia.
Beginnings
Back in the spring of 1793, French colonial refugees, some with slaves, arrived from Cap Français, Saint-Domingue in present-day Haiti. The 2,000 immigrants fled the slave revolution in the island's north. They crowded the port of Philadelphia, where the first yellow fever epidemic in the city in 30 years began. It is likely that the refugees and ships carried the yellow fever virus and mosquitoes. Mosquito bites transmit the virus. Mosquitoes easily breed in small amounts of standing water. The medical community and others in 1793 did not understand the role of mosquitoes in the transmission of yellow fever, malaria, and other diseases.
In the ports and coastal areas of the United States, even in the northeast, the months of August and September were considered the "sickly season," when fevers were prevalent. In the South, planters and other wealthy people usually left the Low Country during this season. Natives thought that newcomers especially had to undergo a "seasoning" and were more likely to die of what were thought to be seasonal fevers in their early years in the region. In 1793 Philadelphia was the temporary capital of the United States, and the government was due to return in the fall. President George Washington left the city for his Mount Vernon estate.
The first two people to die of yellow fever in early August in Philadelphia were both recent immigrants, one from Ireland and the other from Saint-Domingue. Letters describing their cases were published in a pamphlet about a month after they died. The young doctor sent by the Overseers of the Poor to treat the Irish woman was perplexed, and his treatment did not save her.
A 2013 book by Billy G. Smith, professor of history at Montana State University, makes a case that the principal vector of the 1793 plague in Philadelphia (and other Atlantic ports) was the British merchant ship Hankey, which had fled the West African colony of Bolama (an island off West Africa, present-day Guinea-Bissau) the previous November. It trailed yellow fever at every port of call in the Caribbean and eastern Atlantic seaboard.
Epidemic declared
After two weeks and an increasing number of fever cases, Dr. Benjamin Rush, a doctor's apprentice during the city's 1762 Yellow Fever epidemic, saw the pattern; he recognized that yellow fever had returned. Rush alerted his colleagues and the government that the city faced an epidemic of "highly contagious, as well as mortal... bilious remitting yellow fever." Adding to the alarm was that, unlike with most fevers, the principal victims were not very young or very old. Many of the early deaths were teenagers and heads of families in the dockside areas. Believing that the refugees from Saint-Domingue were carrying the disease, the city imposed a quarantine of two to three weeks on immigrants and their goods but was unable to enforce it as the epidemic increased its reach.