Having tested 8,000 oysters from 39 areas across the UK, the Food Standards Agency (FSA) warned on November 29, 2011 that 76% of those tested were contaminated with norovirus, which causes intestinal pain and vomiting, and in extreme cases can be fatal.

http://www.bbc.co.uk/news/health-15927426
http://www.bighospitality.co.uk/Business/Oyster-norovirus-fears-out-of-proportion-says-restaurateur
http://www.medicinenet.com/norovirus_infection/article.htm