Thomas Tsang Ho-fai , the controller of Hong Kong's Centre for Health Protection (CHP), summed up the fears of many in his field about the severe acute respiratory syndrome virus. 'As with other viruses, one can never be certain that it will not return one day, either in the same form or in a different form,' he said.

The head of the University of Hong Kong's department of microbiology, Yuen Kwok-yung, also warned of a lingering threat.

'If you put so many wild animals together in a market in Guangdong it is very likely that it will happen again. But it would not be on such a scale as in the past, so the chance [of a recurrence] is now low,' Dr Yuen said.

Still, he said, 'We must prepare for it. If we prepare for it, we are not only preparing for Sars but for all acute, viral or bacterial respiratory diseases. Unless you are ready when things come, the whole system crumbles, just like in 2003.'

Shigeru Omi, director of the World Health Organisation in the western Pacific, which was at the epicentre of the Sars outbreak, said: 'When the outbreak was declared over in July 2003, the virus had merely retreated back into the animal world.

'In the right circumstances, it may emerge once more. As we have seen with avian influenza and other emerging diseases in the past, the animal world is a constant threat to human health. And it will remain that way until we learn to raise and treat animals in a safer manner.'

An international team of researchers wrote in the journal Nature earlier this year that animals were the source of 60 per cent of the new diseases in humans that they had studied. Of these, 70 per cent came from wild animals.

As a result, health experts believe there are many important lessons that can be learned from the Sars epidemic. 'Perhaps the most important was that public health systems were simply not ready for a disease with such explosive power,' said Dr Omi. 'Attention to communicable diseases had fallen to an unacceptably low level. Infection control measures had become slack and vigilance was poor.

'Now the world is much better prepared for what may come next, whether it is an influenza pandemic or something else.'

Dr Omi also said there was no substitute for transparency and honest reporting.

'In an age where diseases spread at the speed of a jet plane, information must not lag behind. Information sharing is built into the revised International Health Regulations, which stipulate that countries should notify the WHO of all events that may constitute a public health emergency of international concern. So I am confident that the world is much better prepared now than it was in 2003,' he said.

He added that a worldwide influenza pandemic would pose a far greater challenge to public health than Sars.

The CHP's Dr Tsang says that, because of Hong Kong's past experience with Sars, the city would be better prepared when the next public health challenge arrived.

'Nonetheless, we should never be complacent and there always will be unforeseeable challenges and surprises as far as infectious disease outbreaks are concerned,' said Dr Tsang. 'For example, pandemic influenza will be a much tougher challenge than Sars, and when it occurs, the global objective would be on impact mitigation rather than eradication.'

In the years since the 2003 epidemic, Hong Kong's public health infrastructure has been improved through the establishment of the CHP, better communication with the mainland and the availability of more professional expertise.

Dr Tsang said he believed the CHP's main functions were to devise outbreak preparedness strategies, formulate detailed contingency plans and later to test the effectiveness of such plans through drills and exercises.

'Needless to say, a lot of work needs to be done on liaising with other stakeholders in the process, such as the Hospital Authority, various government bureaus and departments, the private sector and the general community,' he said.

Dr Tsang agreed that many of the lessons learned from Sars would be useful in the case of an avian flu pandemic. 'The first priority is global intelligence gathering about avian flu. We must know the first signs of avian flu getting more transmissible between humans, no matter where this occurs,' he said.

Meanwhile, an example of closer communication with China occurred this week when mainland health authorities confirmed a migrant worker in Guangdong died of bird flu on Monday.

Dr Tsang said on Monday that outbreak preparedness was also in place, as public hospitals would test patients with flu-like symptoms of unknown origin who had visited Guangdong in the past six months.

Dr Tsang said that infection control training and preparedness, especially in health institutions, had been overhauled since the Sars outbreak and that space for more than 1,000 isolation beds had been created.

He said that, in the event of an outbreak of pandemic disease, Hong Kong, as the place which, after the mainland, was the worst affected by Sars, could pass on to other cities the valuable lessons it had learned about the use of quarantine and isolation, treatment, health checks at air, land and sea borders and investigating outbreaks.

Scientific studies by Hong Kong researchers had also greatly contributed to the understanding of Sars in terms of its virology, epidemiology and clinical aspects.

Dr Tsang, who in 2003 was a consultant at the Health Department, said Sars had meant a lot to him on a personal level.

'The thing that struck me most was how our health care colleagues, the government and the whole community were so united and dedicated to getting Sars under control by whatever means and at whatever price. The sense of single-mindedness and unity of purpose was seldom so evident,' Dr Tsang said.

'While I was working my guts out in field investigations, meetings and operations, I knew many colleagues in hospitals were working even harder.

'Many people made sacrifices in terms of personal time, families and even their own lives. I think I saw the best qualities and virtues of Hong Kong people as a result of the Sars experience,' he said.

Experts in Hong Kong would like research into Sars to continue, including determining the source of the Sars coronavirus.

'I think the real source of this virus is still a mystery. Although some animal species have been found to be related, they probably are not the real source of the virus,' said Chinese University professor of microbiology Paul Chan Kay-sheung. 'There are still a lot of things we do not know. We are still at risk of re-emergence of the Sars at any time.'

Medical sector legislator Kwok Ka-ki said: 'What we are uncertain about is the [efficiency of the] reporting system between here and China.'

Dr Kwok, a urologist in private practice, also expressed concern over the preparedness of frontline health workers.

'I am not certain if frontline workers are being trained on infection-control measures,' said Dr Kwok. He said the city seemed to have 'geared down'.

'I think we are lucky that we have not had any outbreaks [since 2003]. The test will come some time when we are not prepared. We should remain attentive, try to do the drills as much as possible,' he said.

'Not to the extent of being pessimistic, but I am still unable to say we are prepared for Sars.'

Sian Griffiths, co-chairman of the Sars Expert Committee and director of Chinese University's school of public health, said Sars was a 'wake-up call not only for Hong Kong but the world about how important it was to have effective surveillance systems for infectious diseases'.

'What Sars taught us was the importance of good surveillance, the need for good communication and the need for good collaborative rather than competitive scientific research in a time of crisis. It also showed us the strengths of our health-care system,' she said.