SER-109 Versus Placebo to Prevent Recurrent Clostridium Difficile Infection (RCDI)

Inclusion Criteria: 1. Signed informed consent, indicating that the patient understands the purpose of and procedures required for the study. Patients who are unable to provide informed consent will not be included in the study. 2. Male or female patients ≥ 18 years. 3. ≥ 3 episodes of CDI within the previous 9 months, inclusive of the current episode with documentation of ≥ 2 episodes. Exclusion Criteria: 1. Female patients who are pregnant, breastfeeding, lactating, or planning to become pregnant during the study. 2. Known or suspected toxic megacolon and/or known small bowel ileus. 3. Active irritable bowel syndrome with diarrhea within the previous 12 months. 4. Major gastrointestinal surgery (eg, significant bowel resection or diversion) within 3 months before enrollment (this does not include appendectomy or cholecystectomy) or any history of total colectomy or bariatric surgery. 5. History of inflammatory bowel disease (ulcerative colitis, Crohn's disease, microscopic colitis) with diarrhea believed to be caused by active inflammatory bowel disease in the past 24 months. 6. Admitted to or expected to be admitted to an acute care facility or intensive care unit for medical reasons (not just boarding). Patients discharged from an acute care facility before Day 1 or residing in nursing homes or rehabilitation facilities may be enrolled. 7. Concurrent intensive induction chemotherapy, radiotherapy, or biologic treatment for active malignancy (patients on maintenance chemotherapy may only be enrolled after consultation with medical monitor).