For centuries, humans coexisted with intestinal worms, only recently being widely dewormed. This deworming process was generally considered beneficial due to the negative health impacts of parasitic worms. However, scientists are now exploring the potential downsides of being worm-free, particularly concerning chronic diseases. This episode of Radiolab delves into the American hookworm, Necator americanus. The hookworm's history is traced from a parasitic demon to a potential medical treatment. The program revisits past stories and includes new research on hookworms' relationship to autoimmune diseases. The episode features reporting by Pat Walters and Molly Webster, with production by Matt Kielty and editing by Arianne Wack. The episode also includes a remix of Mozart's Piano Concerto No. 21 by Ethan Hein and thanks to Doris Pierce, and Dan and Alice Hadley. The show provides citations for the research and encourages listeners to subscribe to their newsletter. The program is supported by listeners, as well as several foundations.