Seven people, some of them children, have been diagnosed with E.coli 0157 in a single village.
A further eight people who share the same private water supply are also being tested for the infection.
The water is shared by eight homes in Auchinclech, near Westhill in Aberdeenshire.
Two of the affected people were treated at Aberdeen Royal Infirmary and have now been discharged. The remainder were seen by their GPs.
Health officials at NHS Grampian said the infection was not affecting public water supplies.
A spokesman said: "None of those affected are currently giving cause for concern."
Householders in the affected homes are being advised to boil their water while the inquiry to identify the source of the infection continues.
Dr Helen Howie, a consultant in public health medicine at NHS Grampian, said: "A letter went out to all residents last night to let them know what is going on."
She added that investigators were examining whether heavy rainfall in the area in recent weeks could have led to the outbreak.
Dr Howie said: "This outbreak highlights the risks associated with private water supplies.
"Such supplies are liable to bacteriological contamination causing infections such as E.coli O157.
"Householders are encouraged to contact the Environmental Health Service and seek advice on how to improve their supply."
E.coli O157 is most commonly found in the intestines of cattle.
Humans can become infected by contaminated water or foods, inadequately cooked meat and dairy products, direct contact with animals and by person-to-person spread.
Symptoms include nausea, vomiting and diarrhoea.
In 1996 the O157 strain of the bug killed 21 elderly people in Wishaw, Lanarkshire, during the world's worst recorded instance of E.coli poisoning.



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