Summit is developing ridinilazole as a novel antimicrobial for Clostridioides difficile Infection (CDI), formerly known as Clostridium difficile Infection, with the goal of demonstrating an improved Sustained Clinical Response rate in subjects treated with ridinilazole as compared to subjects treated with vancomycin. A phase 2 proof of concept study, with vancomycin as comparator, demonstrated these attributes with a comparable safety profile. A high fecal concentration of ridinilazole and little systemic exposure were noted. The rationale for this phase 3 study is to confirm the improvement in sustained clinical response of CDI over vancomycin and to compare the safety and tolerability of ridinilazole to that of vancomycin. Ridinilazole plasma concentration will be assessed in a subset of patients.