It's thought thousands of people have visited the children's farm since the first case was confirmed on 27 August.
Doctor explains E.coli
E.coli can live in your body for up to two weeks, so there is a chance that many more people could have it.
The manager of the farm, Richard Oatway, says they acted responsibly.
"Our main priority has always been to make sure the farm is safe for everyone who comes here to visit," he said.
"All the staff at the farm are very upset about the E.coli outbreak and we hope that all the children make a full and speedy recovery."
Vomiting and diarrhoea
Farm manager Richard Oatway's full statement
E.coli causes vomiting and diarrhoea and, in very rare cases, can kill people.
Health officials said there had been 36 reported cases of E.coli so far, in what one spokesman described as one of the largest seen in the UK.
Mr Oatway said he didn't know when the farm would reopen, adding: "We will not reopen the farm until we are satisfied that we have got to the root of the problem and have put in place suitable control measures."