Health experts say 150 children and staff at a Lurgan nursery have been screened following an outbreak of E-coli.
Holly House nursery in Dollingstown voluntarily closed after twelve toddlers contracted the illness.
An industrial cleaning operation is underway at the premises and an investigation is being carried out, but it is understood that none of the children are seriously ill.
Dr Neil Irvine, Consultant in Health Protection at the Public Health Agency said: "The measures have been in place since Wednesday."
"We responded as soon as it was reported to us so I just want to reassure people that measures are in place, and they've been ongoing since it was reported.
"The screening process for children is now well underway."
Dr Irvine added that there was no evidence at this stage as to the source of the infection.
© UTV News