The main objective of this study is to see the number of volunteers that get sick from a Norwalk virus (NV) inoculation with typical symptoms of nausea, vomiting, and diarrhea. If this NV preparation is shown to cause Norwalk illness, then it can be used to test new vaccines in the future. Additionally, researchers hope to determine the lowest dose of NV inoculum that can be given to volunteers and cause illness. Study participants will include 57 healthy adults, ages 18-50. Participants will either be given the NV inoculum or placebo (water without virus). Participants will be hospitalized for a maximum of 7 days and a minimum of 4 days following the NV challenge. Study procedures include physical exam, blood testing, and collection of saliva, urine, and stool samples. Participants will be involved in study related procedures for up to 180 days.