A norovirus outbreak at a festival in south-west Germany has affected more than 800 people.
They caught the vomiting bug in a marquee at the Stuttgart spring festival last weekend.
It is not clear how the virus was first contracted, but the scale of infection has increased during the week.
Stuttgart officials believe it was not linked to food or drink served in the festival tent as samples taken have all tested negative.
The city's health department also tested marquee staff, as well as dishes and water used for washing them up.
Instead they are assuming the virus was passed from person to person, possibly through the air, although it is not clear whether the original source was a visitor or an employee.
Visitors quickly complained of vomiting, nausea and diarrhoea. By Friday afternoon, Stuttgart health officials said the number of people infected had reached 815.
The spring festival, on the banks of the Neckar river, opened last Saturday and runs for 23 days into next month, with a variety of rides, stalls, snack bars and marquees offering refreshments and entertainment. Last year it attracted 1.4 million visitors.
The outbreak has been sourced to the Göckelesmaier marquee, whose operator Karl Maier told German media that "someone apparently brought norovirus in on Saturday evening" - suggesting it could have been an infected group.
Stuttgart health officials say there is no evidence of hygiene rules being broken. The Göckelesmaier marquee was thoroughly disinfected after last weekend and has since reopened.
City spokesman Sven Matis told public broadcaster SWR that there were increasing indications that the virus spread in the central serving area before making its way through the entire marquee.
It is thought most people fell ill after visiting the tent, and some of those infected were working at the time. Officials said it was possible that some visitors may have passed the virus on through secondary infection.
Norovirus symptoms
The main symptoms of norovirus according to the UK's NHS are:
- feeling sick (nausea)
- diarrhoea
- being sick (vomiting)
- a high temperature
- a headache
- aching arms and legs