Noroviruses, illustration. Norovirus or 'winter vomiting bug' is a genus of RNA virus that causes many cases of stomach upset. The disease is characterised by nausea, vomiting, diarrhoea and abdominal pain. The diarrhoea results in fluid loss and dehydration, which may become life-threatening in the young, the elderly, and the immunocompromised if not treated promptly. It occurs usually in wintertime but not always.