Poorly customers in Devon sent questionnaire
More than 100 people who say they have been ill are being questioned by environment health officers to see if it is linked to a Devon diary.
The health experts are trying to establish whether Dunn's Dairy at Drewsteignton was the cause.
Public Health England said products from the dairy had been affected by either cross-contamination or ineffective pasteurisation.
The dairy said it would not comment while the investigation continued.
Questionnaires have been sent to 150 people as part of the investigation into the incident which may be the result of problems with milk processing at the dairy.
The contamination was discovered last month during a joint Public Health England and West Devon Borough Council investigation following an inspection of the dairy which processes about 8,000 litres of milk a week.
Some of the people who consumed the milk are also considering legal action.
Environmental health officer Pete Wearden said at the time a number of cases of campylobacter were being investigated.
Symptoms of the bacteria include diarrhoea, cramping, abdominal pain and vomiting between two to five days after exposure.
The dairy supplies milk to doorsteps and businesses across the county, including West Devon, Teignbridge, Exeter, Mid Devon and Torbay.
The dairy should not to be confused with Dunn's Dairy of Underdown Farm, Exbourne, Public Health England said.