Some 430 people have now reported falling ill with a stomach bug after visiting a spring folk festival in Stuttgart in south-western Germany, a municipal spokesman said on Thursday.
There had been 400 cases reported on Saturday and a further 30 on Sunday, he said. Earlier reports had referred to 300 infected people. The authorities are assuming that many more have been affected, as ailments of this kind can easily be passed on within the family.
All those infected had visited the same tent and had later reported symptoms including vomiting, nausea and diarrhoea, the spokesman said.
The highly contagious norovirus has been confirmed in three cases. But the virus has not been found in tests of food, cutlery and crockery. The authorities are assuming person-to-person transmission.
Immediate action by food monitors and health officials had prevented further spreading, the spokesman said. The operators of the affected tent were doing all possible to ensure hygiene and to prevent a flare-up.
A spokeswoman for the operators of the tent told the Heilbronner Stimme local newspaper that the assumption was that the outbreak could be attributed to infected guests. "We are working cooperating closely with the city and the food authorities," she said.
There was no evidence that a staff member could have introduced the virus. "Only staff that are not showing symptoms and who have confirmed this in writing are being employed," she said.
Noroviruses cause nausea, vomiting and diarrhoea. They spread rapidly, especially in places where many people come together, such as nurseries, old people's homes or hospitals.
An infection is usually short and severe. Those affected feel weak, often have abdominal pain, headaches and aching limbs, and sometimes a slight fever.