1793 Philadelphia yellow fever epidemic immune to the fever. Many of the slaves in Charleston in 1742 could have gained immunity before having been transported from Africa, by having been exposed to yellow fever in a mild case. People who survived one attack gained immunity. A total of 240 blacks died, in proportion to their population at the same rate as whites. Given the limited resources and knowledge of the times, the city's response was credible. The medical community did not know the natural history of yellow fever, a viral infection spread by the "Aedes aegypti" mosquito. Efforts to clean the city did not defeat
Yellow fever vaccine group established in 2000 when a global shortage of yellow fever vaccine had developed because of the long lead time to manufacture the vaccine. As of 2013, there were four WHO-qualified manufacturers: Bio-Manguinhos in Brazil (with the Oswaldo Cruz Foundation), Institute Pasteur in Dakar, Senegal, the Federal State Unitary Enterprise of Chumakov Institute in Russia, and Sanofi Pasteur, the French pharmaceutical company. Two other manufacturers supply domestic markets: Wuhan Institute of Biological Products in China and Sanofi Pasteur in the United States. Seth Berkley, the chief executive of Gavi, has warned about the worldwide threat of yellow fever because of