Viral infections, particularly those caused by noroviruses (NV, genus Norovirus), are the most common cause of community-acquired gastroenteritis in Europe, with respect to both endemic and epidemic occurrence. For the first time, a general practitioner-based case-control study was performed between July 2001 and July 2003 in the German-speaking part of Switzerland in order to identify risk factors for sporadic NV infections. The consumption of different foodstuffs and of bottled mineral water did not show any significant association with the risk of NV gastroenteritis, nor was there any significant effect of individual ABO histo-blood group or household size on the incidence of NV gastroenteritis. The findings are consistent with person-to-person transmission as the most important route of transmission for community-acquired, sporadic NV infection, in that 39% of all patients reported they had had contact with ill persons before their illness. The fact that 33% reported contact with ill persons, mainly within family groups, after their own illness suggested that a substantial proportion of patients were part of family mini-outbreaks.