Matilda "Mattie" Cook is a young girl living in Philadelphia with her mother and grandfather if 1793. In the first chapter, which takes place on August 16th, Mattie wakes up on a very hot morning with a mosquito buzzing around her ear.
This opening scene provides significant foreshadowing of the events that are about to occur throughout the rest of the book. It is not long before Mattie's mother, Lucille, falls ill, as well as Mattie herself. The sickness they are afflicted with is Yellow fever, a viral disease which results in serious symptoms, including bleeding, kidney issues, fever, chills, muscle pains, and even death. It is spread through the bites of infected mosquitoes.
To provide a bit of context of how serious this epidemic was, consider that in Philadelphia (a city of 50,000 people in 1793), at least 5,000 individuals died. This is one of the worst outbreaks in the history of the United States.