In 1793, Philadelphia was hit with a yellow fever epidemic which eventually killed 10% of its population. Fever 1793 is narrated by 14-year-old Mattie Cook as the fever takes over the city and sickens the loved ones around her.
This reminds me of a book that I would have read in the 5th grade. It's historical fiction and it is definitely appropriate for the upper-elementary crowd. It would be a great book for kids to read when they study this time period in US History. As for my adult readers out there, this may not be up your alley, but it may make great reading for your niece.
An exciting note: I just discovered a Little Free Library in my neighborhood as I was walking around last week. I am excited to leave this book there for another reader to pick up!