Twelve children are in hospital - four seriously ill - after contracting the vomiting bug E.coli in an outbreak at a children's farm.
All the children are aged under 10. The Health Protection Agency says 36 cases have been reported so far.
The children are thought to have caught the bug at Godstone Farm in Surrey, which also has a playground.
The attraction receives up to 2,000 visitors a day in the summer. It has been temporarily closed.
The outbreak is believed to have started on 8 August. The bacteria causes diarrhoea and can lead to kidney failure, especially in young children. It is fatal in very rare cases.
The Health Protection Agency is investigating the outbreak and spokesman Dr Graham Bickler says it is one of the largest seen in the UK.
He said: "The farm is shut so we're confident there's no more transmission going on at the farm."
He added it would take "a few weeks" to find the source of the infection: "The actions over the next few weeks will be to investigate it by talking to people who've been ill or their parents to find out what they've been doing, by taking samples from them and from the environment.
"So it will take a few weeks till we fully understand it and we don't know at the moment."
The director of the Surrey and Sussex Health Protection Unit, Dr Angela Iversen, said the farm would remain closed for the time being.
She said: "The farm owners are cooperating fully and we are working closely with them to investigate the source.
"Our advice is that the farm should remain closed to visitors while this work goes on."
Professor Hugh Pennington has led inquiries into E.coli outbreaks in central Scotland and south Wales. He described the latest one as a "very large" outbreak.
He said: "This is more likely to be an environmental source and we've seen outbreaks on farms of this kind where there are lots of young kids visiting, perhaps patting the animals or maybe just touching surfaces where there's manure."
"But this is a very large outbreak for that kind of setting", he said.
"Most people get better without any particular treatment. But unfortunately some people, some kids do lose their kidneys permanently so this is a very, very serious infection."
A statement on the website for the farm and playground said the children's attraction was currently closed but a sister farm in Epsom remained open.
It said: "Due to an E.coli outbreak, we have closed the farm until we can make sure it is quite safe for you all to visit us.
"Our sister farm in Epsom, Horton Park Children's Farm, is open and they would be happy to see you."