Two children who attend the same nursery have contracted E.coli.
A third case of the O157 strain is also being investigated at the premises in Kirkwall, Orkney but the health board said there is "no strong evidence" that they were infected at the nursery.
One child is being treated in hospital while the other child, described as "a household contact" of the first one, is not showing symptoms. The third child being checked for the infection is at home.
NHS Orkney said other children and staff from the nursery are being screened as a precaution.
Director of public health at the board Louise Wilson said: "It must be stressed that this is purely a precautionary measure. We understand that parents of children who attend this nursery will be concerned and we want to set their minds at rest.
"Parents will be contacted directly by the nursery. If you are not contacted, you do not need to take any action. As ever, anyone with health concerns should contact a health professional."
The E.coli O157 bacterium lives in the gut of animals such as cattle, sheep, deer and goats. It can also be carried by pets and wild birds. In humans, the toxins that the O157 strain produces can cause diarrhoea and kidney failure as well as other illnesses. Young children and older people are said to be at the greatest risk.
Dr Wilson said: "We are looking closely at all possible sources of infection. E.coli O157 infections are often food-related but can also be associated with environmental exposure.
"One of the simplest things that can be done to prevent infection with E.coli O157 is to wash your hands thoroughly after using the toilet or changing nappies, and before preparing or eating food. It is also important to wash your hands after contact with animals or their environment."
In August six cases of the strain were confirmed on the island.