Four people have fallen ill after drinking raw milk produced by a Ceredigion firm.
Public Health Wales (PHW) said it was investigating a link between campylobacter bacteria and Penlan y Mor farm, New Quay.
Those who were ill bought the milk from Aberystwyth farmers' market since 1 June but it was also available at the Royal Welsh Show on 26 July.
Symptoms include persistent nausea, diarrhoea, stomach cramps and fever.
Dr Brendan Mason, consultant in communicable disease control for PHW, said: "It is a very unpleasant infection and more serious complications can occur.
"As in any diarrhoeal illness, washing hands after using the toilet and before preparing or eating food is very important and anyone worried about their symptoms should seek medical advice."
Any remaining unpasteurised milk which has not been consumed should not be used.
Hywel Dda University Health Board, the Food Standards Agency and Ceredigion council are part of the investigation.