 This study will investigate a Clostridium difficile vaccine in healthy adults aged 65-85 years. Each subject will initially receive 3 doses of vaccine on 1 of 2 vaccination schedules. The study will assess the safety and tolerability of the vaccine as well as the subjects immune response to the vaccine. One year after the third dose subjects that did not receive placebo will be randomized to receive a fourth dose. Subjects will be followed for up to 4 years after their third vaccination. 