An official inquiry has been announced into the south Wales E.coli outbreak by Health Minister Brian Gibbons.
There is concern over the rising number of cases at 75, and the number of affected schools has gone up to 29.
Officials said on Friday night that the cases total was likely to go up again.
A meat supplier linked to the outbreak, John Tudor and Son of Bridgend, has been told to stop trading, and any business receiving cooked meat from the firm has been told to withdraw it.
Announcing the inquiry earlier on Friday, Dr Gibbons said he hoped there would not be any further increase in the number of cases in the Rhondda Cynon Taf, Merthyr Tydfil, Bridgend and Caerphilly areas.
Sunday: Seven linked cases confirmed by Rhondda Cynon Taf and Merthyr councils
Monday: Cases rise to 23. Officials warn of further rise
Tuesday: Cases rises to 41 in 19 schools. School canteens ruled out as source
Wednesday: Tally rises to 56 in 25 schools. Outbreak linked with Bridgend food supplier John Tudor and Son
Thursday: Cases rise by 10 to 68. Health officials move to quell public fears
Friday: Health Minister Brian Gibbons orders an official inquiry as casualties reach 75
Politicians in the valleys were demanding an assembly inquiry into Wales' worst E.coli outbreak.
At lunchtime on Friday Roland Salmon of the National Public Health Service confirmed the number of cases had risen overnight from 68 to 75.
He had already described the earlier rise to 68 as "dramatic and discouraging".
In the latest update on Friday night, officials announced that two new schools had been affected - Glyntaf Infants and Brynna Primary. Both are in the worst-hit area of Rhondda Cynon Taf.
In a statement, the outbreak control team said: "It is probable that the number of cases will increase further.
"This is not because the source of the outbreak remains live. It is because, it takes time for the germ to incubate - sometimes more than a week - and new cases are emerging who caught the germs before the control measures were taken at the beginning of the week," it added.
The team - made up of council officers, health and food safety officials - said it had also unanimously agreed that all schools should remain open "because with the control measures in place, it was safe to do so."
The officials said this decision would remain under review, and cooked meat would be reintroduced to schools as new suppliers were found.
The Food Standards Agency (FSA) said any business which had received cooked meat from John Tudor and Son should withdraw it immediately and contact their local authority.
The FSA also asked all Welsh local councils to contact any small to medium-sized business to ensure they have not bought any cooked meat from the company.
The majority of the cases involve schoolchildren, three of whom are seriously ill in hospital.
Meanwhile, an examination of the premises of John Tudor and Son - which supplies schools and council premises with food - continues, although there is no conclusive evidence that it is the source of the infection.
Primary: Abertaf; Blaengwawr; Bedlinog; Pengeulan; Capcoch; Caradog; Upper Rhymney; Comin; YGG Llwyncelyn; Cwmdare; Aberdare Town Church School; Troedyrhiw; Rhigos; Glenboi; Maesycoed; Cwmlai; Hirwaun; Parc Lewis, Ysgol yr Castell; Brynna
Infants: Cwmbach; Penygraig; Cynon; Glyntaf
Secondary: Pen y Dre; St John the Baptist; Archbishop McGrath; Ysgol Pen Yr Englyn; Treorchy
For the company's part, it said it had been in business for 48 years and had had no previous hygiene problems.
Kelyn Williams, manager at John Tudor and Son, said that the company was co-operating fully with investigators.
He said: "On Monday we stopped all deliveries and the plant has been shut ever since. We are working alongside environmental health to find the source of the problem".
In 1999 there were 13 confirmed cases of E.coli at a tourist farm park on Anglesey and in 2002 four children at a nursery in Bangor contracted the bug.
In Wales, the normal annual number of cases is 30.
A helpline on 029 2040 2520 is open until 2100 BST on Friday and will also be open on Saturday and Sunday from 0900 - 1700 BST.