Twelve children remain in hospital today - four of them seriously ill - after an outbreak of the bug E Coli, health officials said.
The youngsters, all aged under 10, are thought to have caught the dangerous O157 strain of the bug after visiting Godstone Farm in Surrey.
The Health Protection Agency (HPA) said the outbreak was "possibly the largest in the UK" to be transmitted by animals.
The agency, which said there has been 36 cases in total, was unable to rule out that more children would fall victim to the bug due to its delayed incubation period.
The farm, which has closed due to the outbreak, is a popular attraction boasting a large range of animals which children can pet and feed. During peak periods it attracts 2,000 visitors per day.
The bacteria causes diarrhoea and can lead to kidney failure and can be dangerous in young children.
The bug can return suddenly and aggressively after a person believes he or she has recovered.
The children are being treated in hospitals across the south of England.
Dr Angela Iversen, director of the Surrey and Sussex Health Protection Unit, said: "This is a large outbreak of this infection.
"The farm owners are co-operating fully and we are working closely with them and with colleagues across health and local authorities to investigate the source.
"Our advice is that the farm should remain closed to visitors while this work goes on."
The outbreak is believed to have started on August 8 with the HPA becoming aware of a large scale problem in late August after a number of cases were traced back to the farm.
Once a pattern was established as numbers of infected children started to rise moves were made to shut the farm down.
Dr Graham Bickler regional director of the HPA, said four of the children were "very seriously ill".
He explained: "It's very likely that the source was animals at the farm, we know this organism is in their faeces but we need to find out how it got into the kids."
E Coli outbreaks typically occur through contaminated food and Dr Bickler acknowledged that the scale of this outbreak via animals was unusual.
He added: "As a farm associated outbreak this is possibly the largest in the UK. It's very large and unusual."
A statement on the farm's website read: "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."
Dr Iversen added: "We are urging parents to follow strict hand washing with their families when visiting these farms.
"Although many parents may carry alcohol gels with them, this should be an adjunct to hand washing with soap and water and not a substitute.
"E Coli O157 is an infection that people can pick up when handling or stroking animals, unless hands are thoroughly washed afterwards to minimise the risk.
"It can also be spread easily from person to person so good hygiene is vital, especially in young children whose hand washing after using the toilet and before eating should be supervised."
Professor Hugh Pennington, an expert in bacteriology, said the strain of the bug could have serious consequences for youngsters.
He explained: "It can be quite dangerous for young children and around 15% do go on to get complications which can effect the brain and the heart as well as effect the kidneys.
"The kidney complications can be quite severe, resulting in long-term damage in some instances and the need to use a kidney dialysis machine.
"There is not much that can be done regarding the treatment, it's often just a case of supporting the patient while they fight the bug."
He said there were hundreds of different strain of E Coli, with the majority being harmless.
E Coli O157, is a new strain with the first case occurring in the UK in the 1980s.
The health protection unit is advising anyone who becomes unwell after visiting the farm on the August 8 date to seek medical advice.