How is yellow fever spreading in Fever 1793?

As is so often the case when people are caught up in a crisis they don't understand, the citizens of Philadelphia blame the spread of yellow fever on outsiders. To be precise, they blame refugees from Santo Domingo (modern day Haiti). But this is ignorant nonsense; yellow fever is transmitted through mosquito bites, not through casual contact between humans. Mosquitoes thrive in hot weather, and Philadelphia is in the middle of a boiling hot summer when the story begins. This proves to be the ideal breeding ground for the little critters, who flit merrily from person to person, spreading this deadly virus for which there is no cure.
As the state of medical knowledge in 18th century Philadelphia is relatively poor by modern standards, people have no idea what's going on. So they fill up the gaping holes in their knowledge with scare stories and myths such as the baseless notion that the virus is being spread by refugees.