Investigations into one of the biggest E.coli outbreaks ever at a UK farm will take weeks and focus on what led to such a large exposure to the bacterium.
Thirty-six cases of E.coli infection linked to Godstone Farm in Surrey have been confirmed, including 12 children. Three are seriously ill in hospital.
The Health Protection Agency (HPA) said the size of the outbreak was unusual.
"We have had cases associated with farms in the past but this is one of the largest," said Dr Graham Bickler.
"Something has happened here which has led to a large exposure and our investigation over the next few weeks will try to understand exactly what," Dr Bickler, the HPA's regional director, added.
The outbreak at the farm near Redhill is believed to have started on 8 August.
The E.coli 0157 bacteria causes diarrhoea and can lead to kidney failure, especially in young children. It is fatal in very rare cases.
The first case officially linked to the farm was confirmed on 27 August but the farm's managers were not told that contact with high-risk animals should cease until 3 September.
On Friday, environmental health officers advised the farm to shut the next day.
Andrew Hawkins, from Southborough, Kent, who visited on Friday with his 23-month-old son Ted, said it was unforgiveable his family's health was put at risk.
"I feel so sorry for the parents of the children that are infected already and are seriously ill and our thoughts are certainly with them at the moment," he said.
"In the next seven or eight days we will be on a constant vigil with my son and just pray to God that he doesn't contract anything."
Tracy Mock, 39, from Paddock Wood, Kent, said her two-year-old twin boys were taken ill after visiting the farm on 31 August.
"If they had just shut the place down to investigate, my sons would not be in hospital on kidney dialysis machines," she said.
"They are still in hospital, my partner and I are taking turns to be there with them. One has had a blood transfusion. The doctors say it could be weeks until they are in a fit state to come home."
Dr Bickler said the HPA only knew with hindsight that the first case of E.coli was exposed to the bacteria on 8 August.
There was an incubation period for the disease and testing for E.coli took time, he added.
"It is a serious illness and people are rightly very concerned," he said.
"The point I'd like to make is we can only act on what we know at the time.
"It seems to me to be quite clear that we did the right thing and gave the right advice on 3 September based on what we knew at the time."
Liberal Democrat health spokesman Norman Lamb said he would be writing to HPA chief executive Justin McCracken to demand an urgent review into how the case was handled.
"Given the viciousness of E.coli, it seems extraordinary that the Health Protection Agency didn't take this more seriously and act more quickly," he said.
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