LONDON (AFP) - Three-quarters of British-grown oysters contain norovirus, a bug which causes diarrhoea and vomiting, according to new research published on Tuesday by the Food Standards Agency (FSA). But the government health experts warned that their tests did not reveal the extent of infectious norovirus, the strain that makes people ill. Nor could they say how effective various treatment processes before and after harvesting were in getting rid of the virus, explaining that their study will contribute to a Europe-wide review of the problem. The two-year study looking at samples from 39 oyster harvesting areas across Britain found 76 percent off oysters straight off the bed contained norovirus. It is difficult to assess the potential health impact of these findings, as the available research techniques are not able to differentiate between infectious and non-infectious norovirus material within the oysters, the FSA said in a statement.