Safety and Efficacy of Fidaxomicin Versus Placebo for Prophylaxis Against Clostridium Difficile-Associated Diarrhea in Adults Undergoing Hematopoietic Stem Cell Transplantation (MK-5119-001)

Inclusion Criteria:

* Male or female 18 years of age or older.
* Females of childbearing potential must be using an adequate and reliable method of contraception (e.g., abstinence, barrier with additional spermicide foam or jelly, intrauterine device, hormonal contraception). Males and females must agree to avoid conception during treatment and for four weeks following the end of study treatment.
* Is undergoing HSCT with planned Fluoroquinolone prophylaxis.
* Informed consent is provided.

Exclusion Criteria:

* Ongoing active CDAD infection (as evidenced by clinical signs of diarrhea along with the presence of either toxin A and/or B \[or their respective genes, tcdA and/or tcdB\] of C. difficile in the stool) or current treatment for CDAD.
* Undergoing cord blood transplants.
* Has fulminant colitis, toxic megacolon, or ileus.
* A history of inflammatory bowel disease (ulcerative colitis or Crohn's disease).
* Women who are pregnant or are actively breast feeding (all women of childbearing potential must have a negative pregnancy test result prior to dosing study drug).
* Use of any drugs potentially useful in the treatment of CDAD (e.g. oral Vancomycin, Metronidazole, oral Bacitracin, Fusidic Acid, Rifaximin, and Nitazoxanide).
* Any other condition that, in the opinion of the investigator, would jeopardize the safety or rights of the participant in the study, would make it unlikely for the participant to complete the study, or would confound the results of the study.
* Participation in other clinical research studies utilizing an investigational agent within one month prior to screening and during the study treatment period.