Study of Different Formulations of a Clostridium Difficile Toxoid Vaccine Given at Three Different Schedules in Adults

This study will further evaluate the ACAM-CDIFF™ vaccine in a population of middle-aged to elderly individuals at risk of exposure to Clostridium difficile because of impending hospitalization or residence in a care facility. Primary Objectives: * To describe the safety profile of subjects in each of the study groups. * To describe the immune responses elicited by toxoid A and toxoid B of subjects in each of the study groups. Observational Objective: * To describe the occurrence of first-time Clostridium difficile infection (CDI) episodes.