Pupils have been sent home from two schools in Devon with suspected norovirus.
About 45 children and 15 staff have come down with a bug at Broadclyst and Westclyst Primary Schools, which are on the same site in Exeter.
Headteacher Jonathan Bishop said the spread of the illness had “escalated very quickly” on Wednesday.
Public Health England (PHE) is investigating alongside Devon County Council.
The health body said about 60 pupils and members of staff were currently affected by a bug, which is suspected to be norovirus.
There are about 600 pupils at the two schools and a few children felt unwell on Tuesday, so bosses contacted PHE, Mr Bishop said.
When children were taken ill on Wednesday they were seen by a school nurse before being sent home “to prevent the spread”.
The headteacher said the schools had been deep-cleaned overnight and the outbreak was being taken “very seriously”.
No children had been sent home with the bug on Thursday, Mr Bishop added.
PHE health protection consultant Nick Young said: “To reduce the risk to others, stay off nursery, school or work, avoid preparing food and minimise direct contact with others until 48 hours after your symptoms have stopped.”