Moreover, it’s unclear exactly how the introduction of donor microbes affects the composition in RCDI patients’ guts. “We cure the patient, but we don’t know what’s happening,” said Josbert Keller, a gastroenterologist at Hagaziekenhuis Hospital in The Netherlands, who was not involved in the work. “It’s a completely unstandardized treatment. . . . We need to understand the mechanism of action of donor feces to see what changes and, ideally, we should understand what changes are really needed to cure the patient.”