Parents at a south Wales valleys infants school have begun testing their children for E.coli after four cases of the disease were discovered.
Abercynon infants remained closed on Sunday after health officials could not trace the source of the infection.
The school in Rhondda Cynon Taf had not previously been involved in Wales' worst outbreak of E.coli infection.
Education Minister Jane Davidson said she was "very disappointed" that the fresh cases had emerged.
Ms Davidson told the BBC Politics Show that it was now "really important" that parents tested their children with the kits which were being handed out.
So far 159 people, most of them children, have been affected by the spread of the E.coli O157 bug which first emerged in September.
Another school in the area was closed last month for several days
Five-year-old Mason Jones, from Deri near Bargoed, died last month while a number of children were taken to hospital for treatment for serious complications of the infection.
Abercynon infants was closed on Saturday, and on Sunday parents attended an open session with public health officials at the school to receive advice and information.
They were issued with testing kits so samples could be collected from all 64 pupils at the school, aged from three to seven.
The session took place between 1100 and 1300 GMT. A second session will follow on Monday. The testing process is expected to take at least a week.
One of the first parents to emerge from Sunday's information session was Julie Lane, a health authority worker, with a six-year-old daughter at the school.
She said she had full confidence in the outbreak control team's handling of the situation.
"I think the school is doing the right thing and we're all going to work together," she said.
"People panic, but I think if we keep level-headed and work with the health authority we will get there."
Dr Roland Salmon, director of the communicable disease surveillance centre in Cardiff, said the fresh cases had probably been secondary infections - that is passed from person-to-person rather than from a contaminated source.
Pupils must test clear of the bug before they can return
He said: "I don't think it will be business-as-usual in schools until after the new year."
A 'no-holds-barred inquiry' into the outbreak was promised by the assembly early in the outbreak and on Monday the cross-party committee meets for the first time.
But BBC Wales' Politics Show has discovered that AMs have been told by Wales' most senior civil servant, Sir Jon Shortridge, that they should consider waiting until next spring before beginning the inquiry.
That is because new rules, and not the current draft regulation suggesting how such inquiries should be run, should be in place by then.
It is understood that the civil service believes that having firm rules governing the conduct of the probe could make a legal challenge to its work less likely.
Last month Glenboi Primary School in Rhondda was shut for a few days after evidence emerged of infection among pupils.
A total of 42 schools across the south Wales valleys have been linked to the outbreak.
Technically, Abercynon is the 43rd school involved, but the authorities are not including it in the list because the cause of the infection there is thought to be from a secondary source and not from school dinners as in the other cases.
An investigation led by South Wales Police is continuing into the outbreak following Mason Jones' death.
A meat company which has been linked by health officials to the outbreak is waiting to hear if it can resume trading.
A High Court judge in Swansea reserved his ruling last month on an appeal by John Tudor and Son of Bridgend against a council order forcing it to close.