A two-year-old girl has died and another two children became ill after contracting E.coli O157.
The toddler, from Ballantrae, South Ayrshire, died on Saturday after being taken to Glasgow's Royal Hospital for Sick Children.
Two other children from the Dumfries and Galloway area, whose family had contact with the little girl from Ayrshire, have also tested positive for the E.coli bug.
The youngsters, aged one and three, were admitted to Dumfries and Galloway Royal Infirmary at the weekend but have since been discharged.
Dr Carol Davidson, director of public health at NHS Ayrshire and Arran, said: "We are very sad to confirm that a two-year-old child from South Ayrshire has died in the Royal Hospital for Sick Children, Yorkhill, from complications from E.coli O157 infection.
"Investigations into the source of the infection are ongoing, but at the moment we have no reason to believe that others outside the families affected and their contacts are at any increased risk.
"Our thoughts are with the families involved during this very difficult time."
A spokesman for NHS Dumfries and Galloway confirmed: "A 20-month-old child from Wigtownshire whose family were in contact with the family of the child in Ayrshire was admitted to Dumfries and Galloway Royal Infirmary at the weekend with an E.coli O157 infection.
"The child was discharged from hospital on Wednesday evening.
"A second child aged three years who was also in contact with the family in Ayrshire was admitted at the weekend but was discharged from hospital on Monday."
The board's consultant in public health medicine, Dr David Breen, said further spread of the infection was "unlikely".
He added: "Appropriate control measures are in place." E.coli O157 is most commonly spread by eating contaminated food. Not washing your hands after contact with farm animals is another source of infection.
The incubation period for E.coli O157 is usually around one to 14 days and symptoms can include diarrhoea, nausea, vomiting, stomach pain and fever.