A virus. (NEJM Review)
Eating and drinking contaminated food and water; cruise ships have had outbreaks. Oysters are a good source as they concentrate the virus and the virus is not killed by either freezing or the quick cooking required for a good oyster.
"It is likely that the time and temperature of cooking required to inactivate norovirus in oysters may render the food unpalatable to consumers" (PubMed).
Some strains cause more severe disease.
It is much harder for blood group B to get disease.
The immunoincompetant (transplant, hematologic malignancy) can have prolonged, chronic diarrhea (PubMed).
Different strain have variable morbidity and mortaility with Genogroup 2 Genotype 4 more likely to cause hospitalization and death (PubMed).
And it can be found in harbor porpoises (PubMed). So avoid porpoise sushi.
Diarrhea nausea and vomiting, fevers not uncommon. Asymptomatic shedders are not important for spread of disease (PubMed).
After an outbreak of norovirus, those affected have increased incidence of dyspepsia, constipation, and GERD (PubMed).