Parents at a school closed by an E. coli outbreak will be asked to fill in a questionnaire in a bid to track down the source of the illness.
Redfield Edge Primary School was closed on 20 May and 12 children have been confirmed as having the illness.
The Health Protection Agency (HPA) said parents had reported 40 other children with vomiting and diarrhoea.
The agency will ask parents to answer questions about food and travel habits. The school is set to reopen on 7 June.
An HPA spokesman said children in Reception and Year 1 must be tested and declared free of the infection before they can return.
The agency said it was also aware of diarrhoea and vomiting in children from neighbouring schools - St Barnabas Primary School, in Warmley, and Sir Bernard Lovell School in Oldland Common.
But it said there was no evidence of a connection to the Redfield Edge cases of sickness.
A widespread E. coli outbreak in Europe, which has killed 14 people in Germany, has been linked to cucumbers.