Studies of norovirus infection in human volunteers have not specifically investigated whether the infection spreads beyond the intestine to the local lymph nodes, however, it is possible that systemic invasion occurs in humans with chronic conditions or immunosuppressed hosts [32] [33] [34] [35] [36] . Additionally, viremia has been reported in infections of gnotobiotic pigs and calves [26, 37, 38] . Thus, the ability of MNV to spread to tissues other than the intestine after oral infection may not be unique, but the relationship of this aspect of MNV pathogenesis to human infection is not clear. The availability of strains that can be cleared from WT mice, such as MNV1.CW3, provides an opportunity to define the mechanisms responsible for two cardinal aspects of viral immunity: the capacity to effectively clear acute infection and the immune mechanisms responsible for effective vaccination.