The Norovirus species causes approximately 90% of epidemic nonbacterial outbreaks of stomach flu (or gastroenteritis) around the world. It's a pretty potent virus that was observed for the first time 1968 in children in a school in Norwalk (Ohio, USA).
Currently, the Norwalk virus is the only species within the genus Noroviruses, though some think that a further breakdown will be inevitable in the future. The virus prone to mutating. More and more serotypes are known to exist and are grouped into five genogroups. GI, GII and GIV are the only genogroups known to infect humans. GIII infects cattle and GV infects mice. Porcine noroviruses belong to GII, although porcine and human noroviruses belong to different genotypes within GII.
A virus like this doesn't exist in some sort of vacuum and researchers have been looking long and hard for a host species in the animal world. Recently research was published of 1,077 samples of Noroviruses in oysters. Scientists found that 80 percent of the known human noroviruses matched those found in oysters. The majority of the matches were in oysters from coastal waters and thus more likely to be contaminated with human sewage.
That means that a new and potential circular opportunity to mutate has arisen, because each virus must adapt to a new species in order to survive. People who are sick because of an infection with Norovirus have diarrhea. That is disposed of via sewers and that is often discharged into the sea in areas without proper sewage treatment plants. Oysters filter the seawater and then in turn become infected with the Norovirus. A Norovirus that adapts, mutates and forms a novel subspecies. In the end oysters are consumed raw and the cycle begins anew.
 White: Evolution of noroviru in Clinical Microbiology and Infection - 2014
 Yongxin Yu et al: Molecular epidemiology of oyster-related human noroviruses: Global genetic diversity and temporal-geographical distribution from 1983 to 2014 in Applied and Environmental Microbiology - 2015