Flu is an upper respiratory tract infection. Symptoms include fever, runny nose, sore throat, nausea, vomiting and diarrhoea. Flu viruses are found in many animal species. In humans and pigs, both H1 and H3 flu viruses have been found.

Flu is, by itself, rarely fatal. But it can lead to deadly complications such as pneumonia. A new strain comes up every now and then against which much of the population has no natural immunity, triggering a pandemic.

Swine flu is caused by a virus similar to the type of flu virus that, in various forms, infects people every year, but is typically found only in pigs or in people who have direct contact with pigs.

From December 2005 to January this year, the US Centres for Disease Control and Prevention recorded 12 cases of humans infected with swine flu. Five of the patients had been directly exposed to pigs and six had been near pigs.

'In the past, swine flu had been transmitted to humans and occasionally also it had been transmitted from human to human, but to a limited extent,' Malik Peiris, scientific director of the Hong Kong University-Pasteur Institute, said. 'It has not led to sustained human-to-human transmission until this particular occasion. This seems to be transmitting efficiently and so there is certainly a possibility that it might become a pandemic.'

Outbreaks of swine flu in pigs are known to have occurred in North America, South America, Europe, Africa and parts of East Asia, including China and Japan. 'It does not cause a major problem in pigs so it is not taken very seriously by the pig industry,' Professor Peiris said.

The new flu strain is a mixture of swine flu from North America, Asia and Europe, bird flu and human flu.

'It manifests like ordinary flu but can in some cases quickly deteriorate to pneumonia and death,' Peter Cordingley, spokesman for the WHO's Western Pacific regional office, said. 'We are still working on how it operates.'

In Mexico, health officials suspect it has killed more than 100 people and made some 2,500 ill. People there began falling ill with flu in early February, but authorities initially thought it was seasonal flu.

'From what I understand they thought there was a resurgence of influenza ... it took them some time to realise that this was not just normal influenza but that it was an unusual flu. That really was the problem,' Professor Peiris said.

It was his laboratory in Hong Kong that was the first to isolate the virus responsible for the severe acute respiratory syndrome outbreak in 2003.

On April 17 the US Centres for Disease Control determined that two children in neighbouring counties in southern California were suffering from swine flu. The centre now believes that the earliest onset of swine flu in the United States was March 28.

In Mexico a four-year-old boy in Veracruz state contracted the virus before April 2. Four days later local officials declared a health alert due to a respiratory disease outbreak in the town of La Gloria in Veracruz. By April 24, Mexico had confirmed 20 deaths from suspected swine flu and was investigating another 40 suspected flu deaths.

Schools, museums, libraries and theatres closed in Mexico City, where most of the cases were reported.

On Friday the Mexican capital began an unprecedented five-day shutdown designed to slow down the spread of the disease by creating fewer opportunities for close interaction between people.

More than 2,700 other people worldwide are now suspected to be infected with the virus.

Announcing the decision to raise the WHO's alert level to five, director general Margaret Chan Fung Fu-chun said all countries should 'immediately' activate pandemic-preparedness plans.

She said effective and essential measures included heightened surveillance, early detection and treatment, and infection control in all health facilities.

'Influenza pandemics must be taken seriously precisely because of their capacity to spread rapidly to every country in the world,' she said in Geneva.

Still, she said the world 'is better prepared for an influenza pandemic than at any time in history'.

Hong Kong has been preparing for just such an emergency since the 2003 Sars outbreak. On Friday, Chief Executive Donald Tsang Yam-kuen declared that the city's response level had been raised from 'serious' to 'emergency' following confirmation that a 25-year-old visitor from Mexico had the new flu strain.

The man travelled to Hong Kong via Shanghai on Thursday. He developed symptoms hours after arrival and sought medical treatment at Ruttonjee Hospital. He has since been transferred to Princess Margaret Hospital. The Wan Chai hotel where he stayed has been placed under quarantine lockdown for seven days.

Holiday camps in Sai Kung and Chai Wan have been readied to serve as isolation centres. Districts have been given HK$10 million to buy hand-washing aids and promote good hygiene. The Hospital Authority is ready to open 18 dedicated clinics for swine-flu cases if hospitals become overwhelmed.

Private doctors have been urged to send suspected flu patients to hospital. Travellers arriving by land or air must fill in forms saying where they have been and whether they have flu symptoms. Health officials will discuss enhanced border controls with Shenzhen authorities.

Secretary for Food and Health York Chow Yat-ngok said: 'The chance of controlling and containing this disease is limited, so we will be draconian in our policy. We don't know how damaging or virulent this will be. We don't want it to spread.' He appealed to people who had been in contact with the Mexican traveller to come forward.

Mr Tsang urged people not to panic but to guard against the virus. He said every effort was being made to mitigate the impact of a flu pandemic.

Experts say that Hong Kong, having learned the lesson of Sars - which was caused by an unknown coronavirus strain that infected 1,755 people in Hong Kong, killing 299 of them - is much better prepared for a flu pandemic than it would otherwise have been.

Professor Peiris said: 'I think at the moment in Hong Kong, all the measures that can be taken are more or less in place.'

Tim Pang Hung-cheong, spokesman for the Patients Rights Group, said: 'After the Sars experience both the Hong Kong government and Hong Kong people are now very well prepared for an epidemic.'

The government was 'more vigilant and quicker in responding' to epidemics, he said, and Hongkongers' awareness was quite high. 'A lot of people go to buy masks and are mindful that personal hygiene be kept. They are very conscious about environmental hygiene,' he said.

'Also, the message is quite clear, so people know they have to be very conscious about the epidemic. Up to now I think the government has done a good job,' Mr Pang said.

But infectious-diseases specialist Lo Wing-lok said there might not be enough isolation beds if there was a pandemic affecting a million people in Hong Kong and even 1 per cent of them required hospital treatment. About 1,300 isolation beds have been created in public hospitals.

'We don't have the capacity to cater for a sudden surge of patients to that degree,' he said. 'Whether our system can stand it remains to be seen.'

If there is a pandemic, it will be the first since Hong Kong flu killed an estimated 1 million people worldwide in 1968. The worst flu pandemic began in 1918 and killed tens of millions of people.

US-based flu researcher Henry Niman said the swine-flu outbreak 'has much in common with the 1918 outbreak, including the start in the late spring as a mild infection, the targeting of previously healthy adults, and the origin as swine H1N1'.

'Over the summer the swine H1N1 [virus] will proliferate in the southern hemisphere and recombine with H1N1 seasonal flu, leading to a much more virulent H1N1 in the fall [autumn],' he said.

'Tamiflu resistance will almost certainly develop in swine H1N1, leading to a heavy reliance on a well-matched vaccine,' he said. Tamiflu and Relenza are the principal drugs used to fight flu.

Dr Lo said: 'We should have the expectation that this will have a very serious impact on our community and the government alone won't be able to handle it. The government has to rely on the co-operation of Hong Kong people.

'This is potentially more serious than Sars and Hong Kong people can do a lot.'