- Subjects must be able to demonstrate a sufficient understanding of the study protocol and the ability to follow all required study procedures, including measures to prevent Norwalk Virus (NV) contamination of the environment and spread of NV infection and illness to the community. The prospective subjects must pass (> 75 percent correct responses, or 21 of 27 questions answered correctly) a written examination before enrollment. The exam will contain 27 multiple choice or true/false questions on all aspects of the study protocol. Subjects will be re-instructed about any question they may have answered incorrectly.