 This is phase I, double blind, placebo-controlled safety and infectivity study of experimental human Norovirus genogroup GII.4 administered to healthy adults 18-49 years of age. The unblinded pharmacist will assign treatments. Subjects will be admitted to an inpatient nursing unit, challenged with live GII.4 Norovirus strain by oral administration, remain in the unit for at least 4 days following challenge and then followed for post-challenge safety and efficacy with multiple clinical assessments and collection of blood and stool specimens. 