At least 250 pupils have been struck down by a vomiting virus at a secondary school in Chard, Somerset.
Around one in five of Holyrood School's 1,350 pupils, plus 20 staff members including the head teacher, are off sick with the Norwalk virus.
Symptoms include sickness and diarrhoea for up to 72 hours, and the illness is thought to be very easy to catch.
Parents have been told to keep their sick children at home for at least 48 hours after they have recovered.
The school is staying open and Somerset County Council say it has been thoroughly cleaned since the outbreak on Tuesday.
Paul Courtney, of the Dorset and Somerset Health Protection Unit dealing with the outbreak, said it is believed to have started outside the school.
"With an infection like this it can spread very quickly," he said.