Six cases of campylobacter have so far been linked to people consuming unpasteurised milk from Low Sizergh Barn Farm in Kendal.
The farm said it was co-operating with the inquiry and had suspended sales of raw milk from a vending machine.
Public Health England (PHE) also asked anyone who had bought raw milk from the farm or visited its tearoom in the past two months to complete an online questionnaire.
The farm, which began offering unpasteurised milk from its vending machine in March, sells about 70 litres a day.
In August the farm won a National Trust fine farm produce award for its unpasteurised milk.