The purpose of this study is to compare number of vegetative cells and spores in stool over time for fidaxomicin or vancomycin in patients diagnosed with their first episode of C. difficile infection.

The purpose of this study is to:

• Compare quantitatively the number of vegetative cells in stool over time for fidaxomicin or vancomycin in patients diagnosed with their first episode of C. difficile infection
• Compare quantitatively the number of spores in the stool over time for patients presenting with their first episode of C. difficile infection having been treated with either fidaxomicin or vancomycin.

Inclusion Criteria:

• 18 years of age and above
• First diagnosis of C. difficile infection
• Treatment for C. difficile infection less than 24 hours
• Female subjects must be nonpregnant, nonlactating, and either postmenopausal for at least 1 year, surgically sterile (i.e., documented tubal ligation or hysterectomy) for at least 90 days, abstinent, or agree to use 1 of the following forms of contraception from the time of signing the Informed Consent form (ICF) until 30 days after leaving the study site: a nonhormonal intrauterine device (IUD) with spermicide, female condom with spermicide, contraceptive sponge with spermicide, diaphragm with spermicide, cervical cap with spermicide, a male sexual partner who agrees to use a male condom with spermicide, or a sterile sexual partner.

Exclusion Criteria:

• History of hypersensitivity to fidaxomicin or vancomycin
• Pregnant or breast-feeding
• Active treatment with other therapies with activity against C. difficile
• Receiving any peristaltic agents
• Medical history including ulcerative colitis or Chron's disease
• Ordered to be nothing by mouth or cannot swallow the study medication
• Participation in another clinical research study utilizing pharmacological treatment within 1 month or five half-lives of the medication whichever is longer
• Any other reason felt by the investigator to potentially affect the outcomes of the study

Open-label