An epigraph from Dr. John Snow cautions against using water that has been exposed to waste.

Over the next week, the epidemic slows. Snow credits this development to the removal of the pump handle and the natural progression of the disease. Eel wonders about the original source of contamination, which Snow cautions may remain a mystery. Snow and Whitehead collaborate to further investigate the epidemic; Whitehead is now convinced of Snow’s theory. Snow wonders about the “index case.” Eel believes that it may be Mr. Griggs, but Snow explains that too many people grew sick at the same time for this to be the case. He believes that someone else infected the Broad Street water supply in the first place.

Annie Ribbon’s father, Constable Lewis, dies on September 19. He is the last victim of the epidemic. When Eel visits the Lewises to deliver his condolences, he recalls that the infant, Fanny, was sick before Mr. Griggs. Florrie has now recovered; she and Eel find Whitehead and Snow. When Mrs. Lewis is interviewed, she says that prior to the outbreak, she soaked the sick infant’s diapers in water, which she then dumped into the cesspool below her building.