A two-year-old child has died after contracting the E.coli O157 infection.
The girl who died had been taken to Glasgow's sick children's hospital
The girl, from Ballantrae, in South Ayrshire, died at the weekend after being taken to the Royal Hospital for Sick Children in Glasgow.
A health board spokeswoman said two children in the Dumfries and Galloway area, who had been in contact with the girl, had also tested positive.
A 20-month-old and a child of three were admitted to hospital at the weekend but have since been released.
Investigations into the source of the infections were continuing.
The incubation period for E.coli O157 is usually about one to 14 days and symptoms can include diarrhoea, nausea, vomiting, stomach pain and fever.
A Scottish Executive spokeswoman said: "We are aware of these cases and are being kept informed by the relevant NHS boards.
"Both of these incidents are being managed by the local NHS public health team and Health Protection Scotland who are liaising with us."
Dr Carol Davidson, director of public health at NHS Ayrshire and Arran, said: "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."
A statement from NHS Dumfries and Galloway said: "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 world's worst recorded instance of E.coli O157 poisoning came when 21 elderly people died in Lanarkshire in 1996.
The outbreak, which infected more than 400 people, was traced to John Barr's butcher's shop in Wishaw.
The victims all had the O157 strain.
In May, five toddlers were treated in hospital for kidney failure after an outbreak of the same strain at a nursery in Fife.