Twelve children in Co Armagh have been infected with the E.coli bug, the North's Public Health Agency (PHA) has confirmed.
A total of seven who attend the Holly House nursery in Dollingstown were infected with the potentially dangerous 0157 strain of the bacteria.
The agency said a further five cases were identified after screening. None of the children are seriously ill.
The source of the infection is still under investigation. Holly House has closed temporarily as a precaution until the source is identified.
It said tests on food there had been clear and there was no evidence to suggest the nursery was the cause of the outbreak.
"We greatly appreciate the response of parents which has been overwhelmingly supportive," a spokeswoman said.
"We are now working towards reopening the nursery which we expect to be during the course of this week." Symptoms normally develop after ingesting contaminated foods or liquids or having been in contact with infected animals.
The agency is working with Craigavon Borough Council's environmental health department to investigate the outbreak.
Dr Neil Irvine, consultant in health protection at the agency, said: "The nursery remains closed and all appropriate infection control measures are being taken and the nursery is co-operating fully with the PHA."
PA