Public health officials are investigating a suspected cryptosporidium outbreak linked to a farm shop in Cowbridge.
At least 28 cases has been confirmed after visitors who fed and petted calves and lambs contracted the infection.
Public Health Wales says Cowbridge Farm Shop - based at Marlborough Grange Farm - has voluntarily halted the feeding activities.
Cryptosporidium is a parasite that can cause gastrointestinal illness. It is often linked to contact with animals, especially young farm animals such as calves and lambs.
Su Mably, a health protection consultant at PHW, said: "We are working closely with partners to investigate these cases and to understand if there is a wider risk to the public."
"We would advise anyone who has visited the farm, particularly those who attended feeding or petting sessions, and who is now experiencing symptoms to contact their GP."
“Symptoms typically begin between two and 10 days after contact with the parasite and can last for up to two weeks. Young children and people with weakened immune systems can be particularly vulnerable to the effects of the infection.”
Local authorities including the Cardiff and Vale health board and Vale of Glamorgan Council are also investigating the suspected outbreak.
The main symptoms of cryptosporidium include watery diarrhoeam, stomach pains or cramps, nausea or vomiting, a loss of appetite and weight loss and mild fever.
PHW says it advises anyone visiting farms to follow good hygiene practice, such as washing your hands throughly after touching animals, boots or clothing and before eating or drinking.
They advise parents to supervise children closely to ensure they wash their hands properly while pregnant women should take particular care and avoid contact with newborn lambs during the lambing season.
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