An outbreak of swine flu that has jumped the species barrier from pigs to humans – infecting up to 1,300 people and threatening to cause a global flu pandemic – is likely to arrive in Britain, the scientist who is heading the UK's response said last night.
As the virus appeared to move across Mexico, the US and into other parts of the world, the head of the Health Protection Agency's Pandemic Influenza Unit, Professor Nick Phin said: "If swine influenza continues to spread the way it is spreading, I don't see how we will avoid it.
"If we have one or two people coming in, the chances of it spreading are low but if we get lots of people arriving with it, it could spread very quickly."
Professor Phin said officials were working on the basis that between 15 and 50 per cent of the British population might contract the illness.
The Chief Medical Officer, Sir Liam Donaldson, said four years ago that a flu epidemic was a "biological inevitability" and that it could kill 50,000 to 750,000 people in the UK.
In a day of rapid developments around the world:
*Mexican officials said the disease had killed at least 20 and as many as 86 people, and more than 1,300 people were being tested for infection.
*The US confirmed 20 cases, including eight school children in New York City, as well as cases in California, Kansas, Texas and Ohio. The White House declared a public health emergency. Canadian authorities confirmed four cases.
*In New Zealand, 10 students who had been to Mexico on a school trip were being tested. They are thought "likely" to have the virus, said the Health Minister, Tony Ryall.
*The World Health Organisation (WHO) said the disease could become a fully fledged global pandemic and will meet tomorrow to discuss raising the threat level.
Scientists from the US, Canada and the WHO were called in to tackle the outbreak in Mexico. So far, no cases have been confirmed outside North and Central America.
In Scotland, two people who had returned from Mexico were taken to hospital with flu-like symptoms. In Spain, three people were admitted to hospital.
The WHO disclosed that the flu strains found in Mexico and the US were "essentially" the same virus and declared a "public health emergency of international concern".
Worldwide, health officials are concerned that the H1N1 strain could be as bad as or worse than the last global flu pandemic in 1968 which killed one million people.
A pandemic would deal a major blow to a world economy already suffering its worst recession in decades, and could cost billions of pounds. A new flu strain can spread quickly because no one has natural immunity to it and a vaccine is likely to take months to develop.
At the centre of the outbreak in Mexico City, streets were deserted and restaurants, cinemas and churches closed their doors as millions stayed indoors in an attempt to halt the spread of the illness.
Authorities in Hong Kong, Malaysia, South Korea and Japan checked airport passengers for signs of illness. The UK Government ruled out new screening at airports because symptoms can develop two days after contact.
Officials here are concentrating on surveillance. At the weekend the Department of Health sent out a "public health cascade" to all GPs surgeries, primary care trusts, hospital trusts and other health authorities, advising them to look out for suspicious cases. Officials held emergency meetings in London to discuss the health threat.
If the flu arrives here, sufferers will be asked to remain at home, to prevent the spread of infection. A friend or other person will collect stocks of anti-viral drugs for delivery to the sufferer's home.
The NHS has stockpiled more than £500m worth of the drug Tamiflu – enough to cover half the British population – which has proved effective in Mexico.
The Health Protection Agency is leading the strategy from its nerve centre at the Centre for Infectious Disease in Colindale, north London. "It's being taken very seriously," said Professor Phin.
"The WHO have indicated a Public Health Emergency of International Concern – that's the first time it's been done since the inception of the International Health Regulations. This is exceptional and there's a lot of concern."
Traditional swine flu had three components, he said, but this strain had four: North American avian flu, swine virus, human virus and Eurasian influenza, which is usually found in pigs in Europe and Asia.
Assessing likely mortality from the outbreak was difficult because it had been behaving differently: in the US, no one had died and sufferers were experiencing mild symptoms while in Mexico the disease appeared to be much more lethal.
However, the illness appeared to be very contagious. "In the US they have gone from a situation where at the beginning of April they had half a dozen cases and now there are cases in New York and Kansas," Professor Phin said.
"We could see 15 to 20 per cent of the [UK] population affected, possibly more if they have never experienced the flu. In our planning for events like this we have a scenario where we have assumed a 30 to 50 per cent clinical attack rate."
A government source said Gordon Brown had been informed of the results of an emergency meeting in Whitehall: "The Prime Minister is on top of this. We don't know whether it will get here, but we plan to be prepared. A lot of work has been done over the years in contingency planning for a flu pandemic."
The WHO rates Britain as one of the two countries best prepared for an outbreak, alongside France.
Anyone experiencing symptoms including fever and fatigue after travelling abroad is advised to stay at home and contact the NHS Direct telephone helpline.