The number of confirmed cases of E. coli at a South Gloucestershire school has risen to 12.
Redfield Edge Primary School was closed on 20 May after three children became ill. By Thursday, there were seven confirmed cases.
The Health Protection Agency (HPA) said parents had reported 40 other children with vomiting and diarrhoea.
Two children are currently in hospital and one has been treated and discharged, a spokesman added.
He said there were a further three confirmed cases among family members of the affected children.
Experts are still trying to establish the source of the illness.
The HPA spokesman said the school would reopen on 7 June but children in Reception and Year 1 must be tested and declared free of the infection before they can return.
Sample kits are being delivered to their homes, he said.
Dr David Hunt, from the HPA, said: "The latest total of confirmed cases has risen considerably, but it was expected.
"We've known that a number of probable positive samples have been going through laboratory tests this week.
"The new cases relate to children who reported illness last week, so they do not indicate that the outbreak is spreading right now.
"In fact, there have been no new reported cases of diarrhoea among Redfield Edge children since May 21, which is reassuring, but we continue to monitor the situation closely, not least as spread within families can occur as we've seen from today's figures."
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.