"It came out of nowhere," said Chris Smith, head of Welford Primary, the Birmingham school at the centre of the largest outbreak of swine flu in the UK.
"We followed the (government) guidelines around Catch It, Bin It, Kill It, but we don't know the source - they are still trying to work that out."
The H1N1 virus hit the school during the week before half-term.
Mr Smith said: "We realised something was going on the Monday before half-term. The previous Friday there had been 20 kids off sick out of our 480 pupils.
"On Monday, we had 70-odd off. In one class, only 11 pupils were in out of 31 and the teacher was off sick. We alerted the Health Protection Agency and they went through the process of taking swabs and analysing them. Flu wasn't mentioned until Thursday, and then it was only one suspected case."
Mr Smith decided to close the school for half-term a day early. On Thursday, that single case was confirmed as swine flu. By Monday, six cases had been linked to the school. But it was on the Tuesday of half-term that officials confirmed 44 more cases, sparking headlines that reported parents as saying pupils were "dropping like flies".
Mr Smith spent the weekend in school helping to give out anti-viral drugs to parents, then the half-term week at work. Keeping parents informed and reassured played a key role in tackling the crisis, he said.
The school was deep-cleaned by a team sent in by Birmingham City Council. It reopened on Monday with two-thirds of pupils present and the media camped outside.
Mr Smith said: "I have been a head for 18 years and it has been one of the more challenging weeks."
Swine flu symptoms include fever, fatigue, lack of appetite, coughing and a sore throat. Some have also reported vomiting and diarrhoea. Almost all cases in the UK have been mild and have responded to anti-viral drugs.
But Mr Smith said media coverage of the outbreak had created anxiety. "Viruses do go round schools easily," he said. "None of the children have been hospitalised - all are receiving treatment.
"There is a degree of hysteria around the term 'swine flu'. But the parents have known me a long time. They know I'd not be reopening if children were at risk."
- Last week, it emerged that Eton College would close to pupils, except those taking exams, after seven probable and 25 possible cases of swine flu were identified.
- Who decides whether a school should close?
Usually the head after discussions with the local health protection unit.
- Can exams still go ahead?
Yes, they may be possible.
- What preparations can schools make beforehand?
Schools should ensure they have up-to-date contact details for all staff and parents; that they can keep a sick child separate from other pupils and minimise contact with staff until they can be taken home or collected by a parent; that they are able to stay open if appropriate, and have the means to minimise the spread of infection, such as hand-washing, tissues etc.
- Other points to remember
Pandemic flu is different from ordinary flu because it is a new flu virus that appears in humans and spreads very quickly.
Because it's new, no one will be immune to it and everyone could be at risk of catching it. This includes healthy adults as well as older people, young children and those with existing medical conditions. The best way to protect yourself is through good hygiene. Always carry tissues. Use clean tissues to cover your nose and mouth when you cough and sneeze. Throw tissues in the bin after one use. Wash hands often with soap and hot water or a sanitiser gel.
PANDEMIC ON WAY, SAYS VIRUS EXPERT
A flu pandemic is likely to strike the UK in September or October as children return to school and people return to work after the summer break, a leading expert has warned.
Professor John Oxford, an expert in virology at Queen Mary School of Medicine at the University of London, said the virus would gather strength as it spreads. He said children were usually involved in outbreaks as they like to be in close contact and are often lax on hygiene.