Fifty children and adults connected with a Birmingham primary school are being treated for swine flu.
Forty-four new cases of the disease, linked to Welford School in Handsworth, have been confirmed by the Health Protection Agency.
The agency said that of the total, 45 were children and five were adults, including at least one teacher. The number of cases is expected to rise.
It is the single largest outbreak of the virus in the UK.
The total number of cases in the West Midlands region is now 57 with 184 confirmed cases in the UK.
The 420-pupil school is closed for the half-term holiday. It closed on Thursday, a day earlier than scheduled, after a higher than normal number of illnesses was noted.
A statement issued by the school at the time said the school would have a "deep-clean" during the holiday.
The school was informed on Thursday evening that one of those taken ill was a confirmed case of swine flu.
All parents and staff were contacted and asked to collect anti-viral drugs.
The HPA said all the confirmed cased were being treated at home with anti-virals and were responding well to treatment.
It added there were a number of laboratory tests outstanding and the number of confirmed cases at the school were expected to rise.
HPA regional director Sue Ibbotson said: "While the illness can be unpleasant for those suffering none of the cases have been hospitalised.
"Viruses spread easily in schools and the HPA is working hard with the school and parents to limit its further spread."
She advised that unless people have flu-like symptoms, or are being tested for swine flu, there was no need to stop their normal activities.
Anyone displaying flu-like symptoms should phone rather than visit their surgery.
Donna Pendley, whose nine-year-old son Kenyjah attends the school, said Tamiflu had been given to him, but not to her or her two-year-old daughter.
She said: "We got the leaflet about swine flu some time ago, but when it appears in your area it makes you nervous.
"You wonder how it got to the school, how did it get to Birmingham and Handsworth?
"I am nervous, I really am nervous."