This study will investigate a clostridium difficile vaccine in healthy adults aged 50 to 85 years, who will each receive 3 doses of vaccine. Subjects will receive their vaccine doses at either months 0, 1, and 3 or days 1, 8, and 30. Subjects will be divided into 2 age groups (50-64 and 65-85 years of age). The study will assess how safe and tolerable the vaccine is, and also look at subjects' immune response to the vaccine.

Inclusion Criteria:

• Healthy male and female adults aged 50 to 85 years at enrollment

Exclusion Criteria:

• Proven or suspected prior episode of Clostridium difficile associated diarrhea.
• Unstable chronic medical condition or disease requiring significant change in therapy or hospitalization for worsening disease within 8 weeks before receipt of study vaccine.
• Any contraindication to vaccination or vaccine components, including previous anaphylactic reaction to any vaccine or vaccine-related components