<qa>  What is the main reason for the lack of a human Norovirus?Since human Noroviruses cannot be cultivated in vitro or infect animals, it is necessary to perform human volunteer challenge studies to study the pathobiology of the infection. The study will further the concept that Norovirus has a narrow host range which is fundamental in the pathogenesis and immunology of Noroviruses. Additionally, we recently have identified more than a dozen compounds that specifically block Norovirus attachment to their receptors. The human challenge study should also establish a model that can be used in the evaluation of these new compounds in regards to their capacity to block virus replication anti-viral drug. Furthermore, our study will re-evaluate whether acquired immunity is an independent factor in Norovirus infections which may provide additional information in host immunity potentially useful in vaccine development. Finally, the study will allow for banking of the Norovirus challenge strain by collecting the diarrheal stools passed by the study subjects. Banking of additional specimen, as mentioned above, is critical for future clinical trials because the virus cannot be cultivated in vitro and must be isolated from humans infected with the virus. 