A 72-year-old woman with recurrent urinary tract infections and mitral valve prolapse is seen by her dentist for a tooth extraction. She is given oral clindamycin, 300 mg, 1 hour before the procedure. Three days after the procedure, she develops watery and foul-smelling diarrhea 10 to 12 times per day, fecal incontinence, and abdominal cramps. Her primary care physician orders a Clostridium difficile toxin assay, and the result is positive. She is treated with 14 days of oral vancomycin and improves. Four days after stopping the oral vancomycin, her diarrhea recurs; another 2 weeks of oral vancomycin is prescribed, and she improves again. Five days later, symptoms recur, and she is given a 2-week vancomycin course followed by a 6-week tapered course. Again, she is well on treatment, but 5 days after completing the treatment course, all of her symptoms recur. After 4 months of diarrhea and treatment, she is frustrated, socially isolated, and economically devastated by the costs of her treatments. She comes to see you for a procedure called fecal microbiota transplantation because she has read that it is extremely effective for recurrent C difficile infection (CDI). She asks you why she keeps getting this infection and why this unusual treatment would likely cure her infection.