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In 2008, gastroenterologist Colleen Kelly had a patient with a recurring and debilitating infection of the gut with a microbe called Clostridioides difficile. Nothing Kelly did could ease the woman’s severe abdominal cramping and diarrhea.

So Kelly — at her patient’s urging — decided to try something highly experimental: transplanting a fecal sample from a healthy donor into the large intestine. And it worked.

Kelly, of Brown University School of Me