Project Summary Noroviruses are a major cause of acute gastroenteritis and the leading cause of severe childhood diarrhea globally. Our discovery of murine norovirus (MNV) in 2003 accelerated progress in the field by providing a small animal model and enabling further understanding of the pathogenesis of these enteric viruses. Yet a major limitation of this model system is that wild-type laboratory mice have not been shown to develop overt disease when infected with MNV, in contrast to human norovirus which is symptomatic in immunocompetent hosts. This limits the field’s ability to elucidate norovirus-induced events responsible for disease. It should be noted, however, that all virulence studies of MNV performed to date have used adult mice. Considering that human norovirus infections are more severe in younger hosts, we hypothesized that young mice would likewise be susceptible to MNV disease. We have generated exciting data confirming that oral MNV infection causes acute self-resolving diarrhea in neonatal pups, a transformative discovery for the norovirus field. Gastrointestinal disease severity is regulated by viral genetic determinants and is associated with pathological changes to the intestinal epithelium although the main targets of infection are intestinal immune cells. We propose to use this novel system to test our working model that infection of intestinal immune cells leads to disruption of the epithelium and consequent diarrhea. Our main objectives are to elucidate the viral determinants of diarrhea, define the key immune cell targets of infection, and probe the interactions of the virus with the intestinal epithelium. The integration of these aims will enable us to map key virus-host interactions responsible for intestinal disease.