Answer: Yellow Fever.
Yellow Fever, symptoms of which included high fever, nausea, black vomit, and jaundice, struck Philadelphia particularly hard in the summer of 1793. The first deaths occurred in July, and the fever spread through August, September, and October. Reportedly, local physician Benjamin Rush first encountered the disease in early August, which prompted him to go to Mayor Clarkson to discuss the outbreak. By the time epidemic subsided in November, several thousand residents (between 2000-5000) had died.
Rush and other doctors accurately diagnosed the disease, but he believed that it was the result of putrefied air from spoiled cargo on the docks. The fever was probably brought to the city by Caribbean refugees who were fleeing to America to escape political turmoil in the islands, but then it was spread by mosquitoes that were particularly numerous from ideal breeding conditions that hot summer (The link between yellow fever and mosquitoes was not discovered until the 1890s). While Rush diagnosed the disease, and worked tirelessly to treat the victims, his primary treatment was to bleed the patient. He also believed that African Americans were immune to the fever and he encouraged many of them to assist him with treatment and to cart the dead bodies. Unfortunately, there was no immunity and many of those helpers perished. A hospital for fever victims was established at Bush Hill, Andrew Hamilton’s estate once located near Fairmount Park, which was then occupied by his son William.
Manuscripts and letters on the Yellow Fever can be found in the Maria Logan Dickinson papers (#382), the Pemberton family papers (#484), the Benjamin Rush papers (LCP collection), and others. A published work on the subject is Bring Out Your Dead: The Great Plague of Yellow Fever in Philadelphia in 1793 by J.H. Powell (UPA/Ph RC211 .P5 P6 1965).