Patient description: The patient is a 60-year-old woman admitted to the stroke department with a recent history of a second course of intravenous antibiotics for aspiration pneumonia. She is febrile and complained of abdominal pain and diarrhea (bowel movements eight times in 3 hours, large volumes of greenish, liquid stool each time). The patient's abdomen is generally tender and distend with hyperactive bowel sounds. She is febrile (38.4°C), tachycardic (113/min) and hypotensive (80/40 mmHg). The stool samples must were positive for Clostridium difficile toxin.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Clostridium Difficile Infection participant has a confirmed diagnosis of CDI as defined by: a. diarrhea, as defined by passage of 3 or more loose stools in 24 or fewer hours, AND b. A positive test for toxigenic C. difficile from a stool collected no more than 7 days before study infusion participant must be receiving SOC therapy for CDI. SOC therapy is defined as the receipt of oral metronidazole, oral vancomycin, IV metronidazole concurrent with oral vancomycin, oral fidaxomicin, or oral fidaxomicin concurrent with IV metronidazole participant is highly unlikely to become pregnant or to impregnate a partner since they meet at least one of the following a. A female participant who is not of reproductive potential is eligible without requiring the use of contraception. A female participant who is not of reproductive potential is defined as: one who has either (1) reached natural menopause (defined as 6 months of spontaneous amenorrhea with serum follicle stimulating hormone (FSH) levels in the postmenopausal range as determined by the local laboratory, or 12 months of spontaneous amenorrhea); (2) 6 weeks post surgical bilateral oophorectomy with or without hysterectomy; or (3) bilateral tubal ligation. Spontaneous amenorrhea does not cases for which there is an underlying disease that causes amenorrhea (e.g. anorexia nervosa). b. A participant who is of reproductive potential agrees to remain abstinent or use (or have their partner use) 2 acceptable methods of birth control starting at enrollment and through the 12 Week study period. Acceptable methods of birth control are: intrauterine device (IUD), diaphragm with spermicide, contraceptive sponge, condom, vasectomy and any registered and marketed hormonal contraceptives that contain an estrogen and/or a progestational agent (including oral, subcutaneous, intrauterine, or intramuscular agents) participant or legal representative must have voluntarily agreed to participate by providing written informed consent after the nature of the study has been fully explained  participant with an uncontrolled chronic diarrheal illness such that their normal 24-hour bowel movement habit is 3 or more loose stools participant with a planned surgery for CDI within 24 hours participant has a positive pregnancy test in the 48 hours before the infusion or is unwilling to undergo pregnancy testing if a pre-menopausal female who is not sterilized and therefore has the potential to bear a child participant is breast-feeding or plans to breast-feed prior to the completion of the 12-week study period A female participant who plans to donate ova prior to the completion of the 12-week study period, or a male participant who is planning to impregnate or provide sperm donation prior to the completion of the 12-week study period participant has previously participated in this study, has previously received MK-3415 or MK (either alone or in combination), has received a C. difficile vaccine, or has received another experimental monoclonal antibody against C. difficile toxin A or B participant plans to donate blood and/or blood products within 6 months following the infusion participant has received immune globulin within 6 months prior to receipt of the infusion or is planning to receive immune globulin prior to the completion of the 12-week study period treatment with SOC therapy is planned for longer than 14 days