Short answers, pulled from the story.
Yellow fever is caused by yellow fever virus (YFV), an RNA virus in the family Flaviviridae, and is transmitted primarily through the bite of an infected Aedes aegypti mosquito. The virus can also be carried by other Aedes mosquitoes, including the tiger mosquito Aedes albopictus. In cities, A. aegypti is the main vector; in forests, different mosquito species sustain a sylvatic cycle among nonhuman primates.
Yellow fever begins after an incubation period of three to six days with fever, headache, chills, back pain, nausea, and vomiting. In about 15% of cases, a toxic second phase develops, bringing recurring fever, jaundice from liver damage, abdominal pain, and internal bleeding. Among those who develop jaundice, the fatality rate is 20% to 50%, and severe cases can exceed 50% mortality.
The attenuated live vaccine strain 17D, developed by Max Theiler in 1937, remains the standard protection against yellow fever. The World Health Organization states that a single dose confers lifelong immunity, with protection beginning by the 10th day after vaccination in 95% of recipients. Theiler was awarded the Nobel Prize in Physiology or Medicine for this work.
Carlos Finlay, a Cuban-Spanish doctor and scientist, proposed in 1881 that previously infected mosquitoes transmitted yellow fever rather than direct person-to-person contact. Walter Reed's U.S. Army team, which included doctors James Carroll, Aristides Agramonte, and Jesse William Lazear, confirmed Finlay's hypothesis in the 1890s-1900 period. Reed consistently credited Finlay with the original discovery in his published papers and correspondence.
Yellow fever, transported from West Africa via the trans-Atlantic slave trade, shaped major historical events in the Americas. The disease inflicted an estimated 35,000 to 45,000 casualties on Napoleon's army in Saint-Domingue, contributing to France's withdrawal and the decision to sell Louisiana to the United States in 1803. Haiti declared independence the following year in 1804. In 1793, an epidemic killed more than 9% of Philadelphia's population, forcing the federal government, including President George Washington, to temporarily relocate to Trenton, New Jersey.
Yellow fever had never occurred in Asia until jet travel brought 11 imported cases from the 2016 Angola and DR Congo outbreak. Proposed explanations include that Asian mosquito strains may be less able to transmit the virus, that cross-immunity from related viruses such as dengue may exist in local populations, and that the disease was never historically introduced because the slave trade that carried yellow fever to the Americas did not operate on a comparable scale to Asia. None of these explanations is considered fully satisfactory.