Researchers at Baylor College of Medicine need healthy, adult volunteers to participate in a research study to evaluate a vaccine against Norwalk Virus, a common cause of short-term vomiting and diarrhea illness that occurs in our community (commonly known as intestinal flu or the cruise ship virus). Study requirements involve receipt of two doses of vaccine or placebo in the nose, an inpatient hospitalization (less than one week), and several outpatient visits over a 3 month periods. Participants should expect to become ill for at least a day or more. Criteria for participation are strict. Enrollment is restricted to qualified subjects and participation is associated with some risks. Compensation will be paid according to the degree of a subject's level of participation.