Imagine that the dreaded happens: someone in south-east Asia contracts flu, and also comes into contact with chickens infected with the avian flu virus. The human and avian flu strains mix, mutating into a highly contagious and virulent form that can then be transmitted person to person. It spreads like wildfire and before it is identified, some infected person boards a plane to London. Scores of those passengers, now infected, get onto connecting flights to other UK cities and to other countries, while others get buses and trains into London and across Britain.

The next flu pandemic that experts had warned was overdue has materialised, has an international foothold, and begins killing quickly and indiscriminately. In Britain, the healthcare system and social services crumble under the colossal strain as the number of victims and the rate of spread rise exponentially.

This scenario isn't far-fetched - it's what health experts are hoping to avert right now. The three great flu pandemics of the last century were the result of such flu mutations, the greatest of which claimed 40 million lives in 1918. When you look at the Sars outbreak last year, in the case of Toronto, their first patient infected other patients for five days before the World Health Organisation had named this new, lethal virus and suggested elementary infection control procedures for handling it until more was understood about the disease.

As avian flu spreads throughout yet more south-east Asian countries, medical experts are keeping a close eye on it. The Sars outbreak, which many experts regarded as a stark but relatively merciful "wake-up call", could, they fear, prove to have been mild in comparison.

Dr Jim Young, Canada's commissioner for public safety, steered Canada through the Sars crisis. His hard-earned insights into infection control are now sought by govern ments, health organisations and military groups around the world, to bolster their preparations for an epidemic, the next deadly pandemic, or a bioterrorist attack.

Last week Young was in London addressing a conference on bioterrorism. Despite being jetlagged, he was articulate, focused and engaging as he shared his experiences of the outbreak that claimed 44 lives in Canada and over 900 worldwide within a few months. "We got lucky because Sars was a stern warning, and it cost us a lot of money and 44 lives, but it could have been much worse," he said pragmatically.

Sars was not highly contagious and so community spread was low. It was an ideal hospital-based disease since it required close contact to spread. Once it was better understood, containing it was relatively simple: quarantine anyone with a fever for 10 days and equip hospital staff with gloves, gowns and masks. Easy once you have an understand ing of what you're up against, but days of trying to diagnose patients with an indeterminate illness ended up costing the lives of patients and hospital staff.

And it cost the economy. "The effect on our economy was staggering, absolutely staggering - and immediate," he said. The estimated cost to Canada was $2bn. Avian flu is already costing Thailand and Vietnam dear.

"The key becomes how much, and how, it spreads," said Young. "The great concern about avian flu is that it could mutate and spread human-to-human, and it appears that we don't have great resistance to it - most of those who are getting it are getting very sick.

"If you get something that's spreading as a pandemic then you're getting into a whole series of problems where you run out of resources within the healthcare system - you run out of beds, IVs, and other essential medical supplies. But you also have significant numbers of people who you need to do things who are sick in proportionate numbers - healthcare workers themselves, funeral directors or police officers, or any sector you look at.

"You're trying to manage a big emergency and yet you're losing significant numbers of people. That certainly was a problem with Sars and why it was so important not to get it spreading through a hospital, because it affected so many healthcare workers. Essentially, if you didn't manage it correctly, you took your system down by attrition. And it wouldn't have been restricted to the countries that got it. If it had had community links, it would have spread around the world - everybody would be battling it right now."

If it's going to happen, Young is the type of person you want at the centre of such an emergency. With calls from many for schools and public services to be shut down, and with the economy winded and the healthcare system under enormous and growing pressure, it takes a certain personality to be at the centre of decision-making.

Young was frank but as modest as possible about why he was suited to handling the crisis. "It really does help if you make the assumption that you've got the personality for the task. It's very challenging and stimulating. You don't have any trouble getting up in the morning and going to work. It helps if you have the kind of personality and experience where you make decisions and can go home at night and sleep. A very, very important part of what I do is to keep everybody else thinking positively, focused and upbeat, and not tense. The leader has to portray a calmness, be focused and upbeat, otherwise you and your team are going to make mistakes."

Having highlighted several key gaps in some of the world's best and technologically most advanced hospitals and healthcare systems, Sars took many by surprise. In Canada, protocols with the help of hindsight have been implemented to deal with another outbreak and each stage that it reaches, hospital by hospital. Infection control standards within hospitals are now much stricter: "While there was some degree of that before, it certainly raised the issue and taught us a lot about trying to manage a quarantine in a modern society since it had been 50 years since anyone had last quarantined. Trying to do it 50 years ago and trying to do it now is very different.

"We're paying a whole lot more attention now both to people who are admitted with fever and people who develop a fever in hospital, and making much more detailed assessments of what the fever actually means and whether someone should be isolated until it's figured out what it is."

Sufficient medical supplies and a network to distribute them immediately are now in place to contain any future outbreak. Public health computer systems, to monitor, track and exchange information on patients, have been expanded and improved, and contingency plans, working relationships and communication networks between the police, public health and hospital staff and the media have been established to ensure a more orchestrated and efficient future response.

Perhaps most importantly, strong international links have been formed to pool information, alert other countries of possible outbreaks and fight viruses with the collective expertise of the world's bioscience community.

Would Britain be more suitably prepared for an outbreak of a new, deadly and contagious virus, having observed the lessons of countries such as Canada? Recall how quickly foot and mouth spread in Britain, and that was despite our ability to trace the movements of millions of animals.

Ian Jones, a professor of virology at Reading University, believes that much more can and should be done to minimise our vulnerability. For a start, he says, there is an urgent need for research funding for emerging human infectious diseases: "One of the problems with flavour-of-the-month research is that you take your eye off the ball and look at the re-emergence of smallpox or something, which is actually quite unlikely, and you forget that these historical viruses are much more likely to strike again. You can't really plan for an emerging disease, so I think to have some sort of ring-fenced fund for emerging infectious pathogens wouldn't be a bad idea."

It is also important to ensure that sufficient vaccine stocks are on hand and to establish a system whereby they can be manufactured quickly: "Vaccines can be made extremely quickly from any strain that's isolated - the logistics of doing that on a certain scale ought to be investigated and contingency plans put into place," Jones believes.

A final word of warning from Jim Young: "With emergency planning, the reality is that it's a continuum - you're always trying to improve. You have to battle the complacency that governments and people have, that the emergency's always going to come to someone else, not to you.

"We're better prepared than last year, and next year we'll be better prepared than this year."