Cuba, 1900. This is a MG-level book that explores the discovery of how yellow fever is spread - basically one of those "breaking the codes" that led to numerous public health campaigns and led to more discoveries about yellow fever in the 20th century. Major Leonard Wood of the US Army spearheaded a team of doctors/scientists to look at four theories for the spread of yellow fever - a specific bacteria, the soiled remains (corpses and bedding/clothing) of yellow fever victims, something in the water/air, or contact/bites from a specific breed of mosquitos. The last theory, wild and well-mocked by most scientists around the world, was the brain-child of a Cuban doctor (Carlos Juan Finlay). Turns out Dr. Finlay's theory, down to the correct breed of mosquito, was correct - he only got recognition for this outstanding work by the experiments of Reed's team. Pictures from that time/place help give the reader a sense of environment the doctors were working in.
The book only talked about how the disease is spread; the epilogue goes through a quick time line of the medical advancements in combating the disease (still no cure, but there is a vaccine).
Recommended for upper elementary students (4th - 6th grade) to encourage them to read more non-textbook science/history of medicine books. Would make a great pairing with the MG-level fiction book Fever, 1793 by Laurie Halse Anderson.