The Birmingham adult confirmed among three new cases of swine flu is a 41-year-old Redditch woman, it has emerged.
A 12-year-old girl from Paignton in Devon and a 22-year-old man from south London have also contracted the disease.
A Department of Health spokesman said the Redditch woman had been treated at home and was the only case recorded in the West Midlands.
The Health Protection Agency, which treated the woman, said it had given her anti-virals and that she was "getting better”.
Earlier it emerged a family from Walsall had been tested for swine flu after returning from a holiday in Mexico.
Andrew Lutwyche, his partner, daughter and step-daughter were awaiting the result of tests after one of the children came down with a cough and cold following their return home.
Mr Lutwyche was unavailable to comment on his family’s test results but a spokeswoman for Walsall Primary Care Trust said one result it had back was negative. She was unable to confirm whether it was for Mr Lutwyche’s family.
Meanwhile, an expert on air travel at the University of Birmingham said the swine flu outbreak was potentially very serious given how quickly it has been able to spread.
Dr Pat Hanlon from the university's Business School is an expert in transport economics with a specialism in air travel. He said: “The most striking thing is the speed at which it has been transmitted over huge distances. It was only a few days ago it was actually identified as being an outbreak in Mexico.
"Very shortly after that we were hearing of cases in New Zealand, Europe, America and now there are 23 suspected cases in Scotland. Air travel has played a large role in that.
“In previous major pandemics people weren’t flying around quite so readily and it took longer for the flu to take hold.
“The fact it is spreading so quickly means if it does turn out to be serious then it is going to be a huge problem for health authorities to counteract.”
He said it was reassuring to know passengers returning to Birmingham from Mexico were being checked over.
“Measures like this have to be put in place to minimise and mitigate against this wretched disease spreading,” he said.
The UK's first two confirmed patients, Iain and Dawn Askham, of Polmont, near Falkirk, had been on honeymoon in Mexico. The three new cases, who had also recently travelled to Mexico, were displaying mild symptoms. The 12-year-old girl had been on the same flight back to the UK as the Askhams.
At a press conference at the Department of Health in central London, Health Secretary Alan Johnson said the girl's school, Paignton Community College, was closing for seven days.
He said the Health Protection Agency (HPA) in Devon identified that the girl had been in close contact with 50 fellow pupils and others.
Anti-virals have been given to 230 pupils - the whole of her school year - as a precautionary measure. He said the advice was that schools and children's services should continue to operate as normal.
The World Health Organisation alert remains at "phase four", indicating clusters of outbreaks and that the disease is being passed from person to person.
In the event of a pandemic, the alert will be raised to phase six but Mr Johnson said it was "very clear we are not at that stage yet".
"We've been preparing for this situation for the last five years," he said.
Stockpiles of anti-viral drugs, currently enough to cover 33 million people, are being increased to provide for 50 million people.
Mr Johnson said evidence showed the provision of face masks to the public would do nothing to prevent the spread of the disease. But he said NHS frontline staff would be provided with special surgical masks with filters.
"To keep the public informed, a mass public health campaign will begin tomorrow, with print, TV and radio adverts," he said. "The adverts will warn the public about swine flu and remind people to cover their noses and mouths with tissues and then throw the tissue away."
He said the message was to "catch it, bin it, kill it".
An information leaflet will be posted through people's doors on Tuesday.
Mr Johnson said: "Our philosophy has to be to hope for the best but absolutely prepare for the worst."
He said it was "inevitable" there would be more confirmed cases of the disease.
There would be an HPA presence at every airport where aircraft were arriving from the affected countries and every passenger would receive information about the disease, he said.
Airlines will also have to keep flight records of who is on board those flights beyond the usual 24 hours.
Chief Medical Officer Professor Sir Liam Donaldson said all cases so far had been "imported" from the main affected areas.
An escalation towards a pandemic would come from a "sustained onward transmission" of the virus.
He said the general prophylactic or precautionary use of anti- viral drugs would not be a sensible policy because it would use up supplies of the drugs.
However he said it was sensible for people to take anti-virals if they were directly at risk, having been in close contact with people who had contracted the disease.
He added: "The scientific advice for face masks is that they are of very little value. First of all because they get moist and that enhances the risk of the virus transmitting, and because the virus is so small they can go through the pores. Even more than that they give a false sense of security."
He said it was more important to abide by basic hygiene measures. And he said the development of a vaccine would depend on understanding more about the profile of the virus.