A 46-year-old man presents as an inpatient consult for persistent watery diarrhea, nausea, and abdominal cramps. The patient has a history of acute myeloblastic leukemia and received an allogeneic stem cell transplant 6 months ago. The patient notes compliance with his immunosuppression medications. The most common cause of diarrhea in this patient is most likely caused by which of the following?