The primary objective of this observational study is to estimate the 90-day response rates to treatment for a first episode of C. difficile infections (CDI) in adult transplant recipients. This includes treatment success, recurrence, and complications in a prospectively followed cohort of solid organ and hematopoietic stem cell transplant patients. Patients who develop CDI within 2 years after transplantation will be enrolled and followed prospectively for 90 days after enrollment. Specimens of stool and serum collected within 48 hours of initiation of treatment will be stored locally for future epidemiologic studies to examine the immunologic and microbiologic risk factors for relapse and treatment failure, as well as complications of CDI in the transplant population. Response rates will be classified by organ type, hematopoietic stem cell transplant typ, treatment regimen, and immunosuppressive regimen.