This is an open-label, non-randomized study. The purpose of this study is to better understand how vaccines against typhoid fever affect the normal immune system and bacteria in the intestine. Patients having standard-of-care colonoscopies will be divided into 3 groups:

Group 1: Vivotif typhoid vaccination then colonoscopy; Group 2: Colonoscopy, then Vivotif typhoid vaccination, then follow-up colonoscopy; Group 3: Colonoscopy without vaccination.

The Vivotif typhoid vaccine used in this study is licensed by the Food and Drug Administration (FDA) for travelers to developing countries. Volunteers will be asked to donate tissue, blood, saliva and stool samples for studying how the body responds to the typhoid vaccine.

Inclusion Criteria:

• Age 18 years and older
• Undergoing colonoscopy for screening or surveillance for colorectal cancer at the University of Maryland
• Provide written informed consent prior to initiation of any study procedures
• Healthy, as defined by considered fit to undergo outpatient elective colonoscopy by the referring health care provider

Exclusion Criteria:

• Pregnancy or nursing mother

• Known coagulopathy or bleeding disorder preventing mucosal biopsy

• History of Crohn's disease or ulcerative colitis

• Surgical removal of the ileocecal valve or any part of the small or large intestine

• Allergic reaction to oral typhoid vaccine in the past

• Immunosuppression from illness or treatment, including

  • immune-deficiency disorders such as Human Immunodeficiency Virus (HIV) or Acquired Immunodeficiency Syndrome (AIDS)
  • leukemia, lymphoma, or cancers
  • immune suppressive medications or treatments, such as corticosteroids or radiation

• Receipt of any other vaccine two weeks prior to receipt of Ty21a

• Positive urine pregnancy test (HCG) prior to colonoscopy or vaccination