 Noroviruses are difficult to study because: 1 they are genetically and antigenically highly diverse and multiple strains with distinct genetic identities co-circulate in the same communities, making diagnosis and disease control extremely difficult; and 2 the virus cannot be cultivated in cell culture and 3 no small animal model for studying Noroviruses exists. The molecular cloning of the prototype Norwalk Virus NV in 1990 by one of the co-investigators of this application has allowed rapid progress in studying Noroviruses and other genera; however, many issues on Norovirus infection, pathogenesis, immunology and host range remain unknown. 