A Study to Compare Safety and Efficacy of OPT-80(Fidaxomicin) With Vancomycin in Subjects With Clostridium Difficile-associated Diarrhea (CDAD)

Inclusion Criteria: * Inpatients who have symptoms of CDAD as defined by; * (1)Diarrhea: with ≥4 unformed bowel movements (or ≥200 mL unformed stool for subjects having rectal collection devices) and * (2)Presence of either toxin A and/or B of C. difficile in the stool * Subjects who have not received antibacterials (vancomycin, metronidazole, et.al.) aiming at CDAD treatment before the study Exclusion Criteria: * Life-threatening or fulminant CDAD * Ileus paralytic or toxic megacolon * Likelihood of death before the completion of study from any cause * Concurrent use of oral vancomycin, metronidazole, et.al. aiming at the treatment of CDAD * The anticipated need to continue other antibacterials for a period exceeding seven days from providing the informed consent * Subjects who in the opinion of the investigator require other drugs to control diarrhea * Need of change in dosage regimen of opiates during the study period * Need of change in dosage regimen of probiotic products during the study period * History/complications of ulcerative colitis or Crohn's disease * Multiple occurrences of CDAD within the past three months * Hypersensitivity to vancomycin * Previous exposure to OPT-80 (fidaxomicin) * Female patients who are pregnant, breastfeeding or possibly pregnant, or wishing to become pregnant during the course of study * Participation in other clinical research studies or Post Marketing Clinical Trials utilizing an investigational agent within one month prior to providing the informed consent or within five half-lives of the investigational agent, whichever is longer