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Children and adults connect to the Tri Ceffyl Bach Nursery are being tested for the bug.
Twenty seven people have been tested so far after a child first became ill last Thursday (13th Oct) leading to the nursery being closed.
Less than a week later and Public Health Wales have told Heart that they have confirmed more cases with a control team working to try and find the source of the outbreak.
Dr Chris Whiteside, Consultant in Communicable Disease Control for Public Health Wales says that could be tough: "It can be difficult to identify a definitive source in an outbreak like this because the bug is so easily spread where young children are concerned.
"People can become infected with E. coli O157 by eating contaminated food, or through contact with infected people, farm animals or contaminated water."
Symptoms of the bug range from mild diarrhoea, stomach cramps and fever to severe bloody diarrhoea. People would usually show symptoms within a few days, but it could take up to two weeks.