The Health Protection Agency is investigating an outbreak of E.coli after five cases were confirmed at a primary school.
Redfield Edge Primary School, in Bristol, was closed on May 20 after three pupils became ill.
The children were taken to hospital for treatment, and one was still being treated today.
The Health Protection Agency (HPA) said it was now looking at other primary schools in the area where milder symptoms have been reported.
The HPA said that in the last week parents of 40 pupils at Redfield Edge School had reported mostly milder symptoms in children including stomach ache and diarrhoea, with some experiencing more severe symptoms.
The agency added that all affected children were being laboratory tested for E.coli O157.
The source of the infection is still unknown.
Dr David Hunt, consultant with the HPA's South West Health Protection Unit, said: "We are very grateful to Redfield Edge School for all their assistance in this investigation, and also to the parents of its pupils, who are helping us complete detailed questionnaires, seeking common factors among children with symptoms.
"The incident team's investigations continue. There remains no evidence that the illness was due to food served at the school. It may have come from outside the school and we are keeping an open mind."
The HPA said its team was also aware of cases 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 that these were connected to the Redfield Edge cases of sickness.
Dr Hunt said: "We are also looking into reported cases of diarrhoea and vomiting at neighbouring schools, St Barnabas and Sir Bernard Lovell.
"Testing is under way, and currently there is no indication that these are linked to Redfield Edge, but we are keeping the situation under close review.
"At any time of year, we would expect to see a variety of gastrointestinal bugs, such as norovirus, causing some milder illness in school settings."
An incident team, including communicable disease experts from the HPA, public health colleagues from NHS South Gloucestershire, and environmental health officers from South Gloucestershire Council, are looking at any common links between the children affected to try to establish a source for the illness.
Redfield Edge School has said the earliest it is due to reopen is June 7.
The HPA has advised parents if they are concerned about a child's symptoms to contact their GP.