Bird flu does not sound like one of the greatest threats to Britain and the world since the medieval plagues. We are well acquainted with flu - there's an outbreak, of differing severity, every winter - and, in an age that is increasingly beating back cancer, it seems no big deal. And it is hard to believe that the wings of much-loved migratory birds could bring us a deadly disease, literally out of the blue.

But this has been the week when Britain has realised that it does indeed pose a real peril. Seven days of activity - foreshadowed by the revelation in last week's Independent on Sunday that migrating birds were carrying the virus towards Britain and Western Europe - have brought a vague, far-off danger alarmingly close to home.

On Monday, the Dutch government abruptly ordered all of its free-range chickens indoors to stop them from being infected. Germany, "expecting the worse", got ready to do the same at a day's notice. British officials met farmers to discuss (less drastic) measures. And an emergency meeting of European Union governments agreed on a Europe-wide early warning system to try to spot the arrival of the disease.

It has been rather like the week, nearly 10 years ago, when we suddenly learned that BSE did indeed threaten our health. And, like then, we have good reason to question whether the Government has done enough to protect us.

Like then, too, there have been plenty of warnings. Back in January, Professor John Oxford, one of the world's leading authorities on flu, likened it to "a tsunami rushing towards us". Another, Professor Michael Osterholm of the University of Minnesota, a key US government bioterrorism adviser, added: "I cannot think of any other risk, terrorism or Mother Nature included, that could potentially pose any greater risk to society than this."

The Government's official emergency watchdog agrees. As this newspaper reported at the time, it formally advised a cabinet committee in July that bird flu posed as great a threat as terrorism, and has quietly arranged for mortuaries and emergency services to prepare for up to 750,000 deaths in Britain, should the worst occur. One of the country's top experts, Professor Hugh Pennington, president of the Society for General Microbiology, is even more pessimistic: he told The Independent on Sunday in March that up to two million Britons could perish.

Small wonder that Lee Jong-wook, the director-general of the World Health Organisation, has called this "the most serious health threat facing the world". His top experts have warned that "we are closer to the next flu pandemic than we ever were", and that it is now "knocking at our door".

Pandemics are a quantum leap in the evolution of flu, caused when a new strain suddenly emerges. No human being has any immunity to it, and so - if it is vicious enough - it can kill tens of millions. That is what happened in 1918-19, when a great pandemic carried off 50 million people - including 250,000 in Britain - claiming more victims than the First World War, which immediately preceded it. Smaller, much less serious, pandemics struck in 1957 and 1968. There seem to be three or four of them every century - and experts agree that the next one is overdue.

Ominously, they usually start in wild birds. Waterfowl carry flu viruses, normally without being affected by them, and pass them on to poultry, such as chickens and ducks. These, in turn, affect people. Normally this happens in China or South-east Asia, where many millions of people and domestic birds live cheek by jowl in countless smallholdings and city back gardens.

The present bird flu virus, codenamed H5N1, has so far followed this pattern, and by early this year was firmly established in eight Asian countries. Slaughtering some 150 million birds in the region has failed to eradicate it, and so far 112 people are known to have caught the disease; about half have died.

This is an appalling death rate, some 10 times worse, for example, than in the catastrophic 1918 pandemic. To make matters even worse, its main victims so far have not been the elderly and unwell, the usual targets of normal flu, but teenagers and young adults.

The chance of it turning into a pandemic is, as Professor Pennington puts it, keeping senior officials "awake at night". For this to happen, however, the virus has to go through a crucial mutation so that it passes easily from person to person. So far as we know, this has not yet happened. Most of those who have caught the disease have been directly affected by chickens and other birds, mainly by their droppings.

There seem to be a few cases, as yet not conclusively proven, where people have passed the disease on to members of their families, but the infection has rapidly petered out.

There are also unconfirmed reports that it may have spread more widely to affect hundreds of people in a remote part of northern China. This is denied by the Chinese government, but the reports come from the same sources that disclosed the Sars epidemic when Chinese authorities were attempting to hush it up.

No one knows if H5N1 will turn into a full-blown pandemic or, if so, when it will happen. For that to take place it has to mix with a normal, highly infectious flu virus through a human or an animal - usually a pig, which can get human flu - catching both viruses at the same time. This is a rare event, but the more H5N1 spreads, the more likely this becomes.

Nobody can tell, either, how deadly the resulting virus would be. It would almost certainly lose some of its virulence as it spread among people, if only to enable enough to survive to infect others widely. But Professor Robert Webster, of St Jude's Medical Research Hospital in Memphis, Tennessee, acknowledged as the world's leading expert on bird flu, believes that the pandemic will be "much worse" than the 1918 one. He says: "It is the worst virus I have ever seen."

As well as killing millions, such a pandemic would cripple the world economy, not least because it would target the most economically active people. A report by the Bank of Montreal, published on Thursday, predicted that it could cause an economic collapse unprecedented in modern history. But there is also some good news. This is the first time in history that humanity has been able to predict a possible pandemic, giving us a chance to prepare for it and to blunt its worst effects. The bad news is that we have very few weapons to fight it with. No fully effective vaccine can be developed against a strain that threatens humanity until it actually emerges. Then it will take four to six months, at best, to produce, too late to head off the first wave of the pandemic.

Less effective vaccines are being developed, in the hope that they will take the edge off the disease, while not beating it completely. But nobody knows how well they will work and the world does not have the production capacity to make enough of them.

The best hope lies in some newly developed anti-viral drugs. These, too, are untried in a pandemic, but it is thought that they will save lives and at least slow down the virus's spread. More than 25 countries - including Britain - are stockpiling them, but supplies are limited and those that did not get their orders in long ago will have to wait months for them.

Most countries have done little, if anything, to anticipate a possible catastrophe. The World Health Organisation says: "The world is not well prepared for a pandemic and is not on track to be so even in a year's time." Britain has done more than most, but would still be largely defenceless if the pandemic were to strike in the next few months. It has so far received only 800,000 of the 14.6 million courses of the drug it ordered six months ago, and may have to wait another 18 months to get them all.

This summer ministers also announced that they would lay in two million doses of vaccine to try to safeguard vital workers such as those in the health service. But they have not yet even decided which firm to ask to produce them, and officials can give no prediction of when they will make up their minds.

The Government has drawn up plans to close schools and cancel sporting fixtures to try to slow down the spread of the virus and is about to issue a booklet to GPs with instructions on how to handle a pandemic. But it accepts that, once it reaches Britain, it cannot be stopped from sweeping through the country. Trying to stop people bringing it in by screening at airports will not work as victims are infectious for several days before symptoms appear. And ministers have so far rejected the Dutch move of bringing in free-range chickens to prevent infection from migrating birds.

Perhaps, in compensation, the Government has been tempted to fall back on the old BSE tactic of false reassurance. Last week the Department for Environment, Food and Rural Affairs was insisting that there was little chance that wild birds could bring the virus to Britain, though Dr Bob McCracken, the president of the British Veterinary Association, says it is "inevitable".

Officials at the Health Protection Agency have boasted that Britain's state of preparedness is the envy of the world, despite its deficiencies. And the Department of Health insists on an extremely optimistic assessment of the number of Britons who could die, putting the figure at just 50,000.

Just as with BSE, this is likely to be a grossly counterproductive tactic. Designed to prevent panic and increase public confidence, it is almost certain to have just the opposite effect. And if the pandemic does come, we will need all the coolness and trust in those charged with handling it that we can possibly muster.

Siberia: In the wilderness, not enough petrol to burn the dead birds

By Damian Grammaticas in Gorodische, Siberia

Even before bird flu arrived in Russia, life in this remote village was tough. The farming settlement of Gorodische is 300 miles from the nearest city, in the middle of the vast Siberian wilderness. It's surrounded by mosquito-laden marshes and huge tracts of empty land. Already at this time of year night-time temperatures fall to a chilly 5C.

Gorodische's remoteness hasn't spared it from bird flu. The virus was brought here from China, by the migrating waterfowl that throng the wetlands. The disease began killing domestic birds three weeks ago. Vediney Zagorov has lived here all his life. His yard is empty. All his chickens have gone.

"Nobody knows exactly where it came from," the old man says. " We think it may have come from one type of duck. But who knows? So nobody can guarantee that it is over now." He says the authorities had warned people that bird flu was infecting other settlements in the area. His neighbour's ducks were first to be affected. But she didn't report the deaths.

"She knew the officials would want to slaughter all her flock. She didn't want that to happen," said Mr Zagorov.

When his own chickens began dying, teams of men came in protective suits and masks to cull the poultry in the village. They tried to burn all of the birds, but didn't have enough petrol for the job. So for 10 days a pile of carcasses was left in Gorodische, until locals complained and the bodies were buried.

Further down the street another old man, Alexander Andreyich, is leaning against his fence. They authorities gave compensation of 100 roubles (£2) for each chicken, 150 roubles (£3) for a duck, 200 roubles (£4) for a goose.

Mr Andreyich took the money. "But it's not much good to me," he says. "What are we going to eat when winter comes? It's the meat and eggs that keep us alive." Several other villagers didn't want money, so they killed their own birds and cooked the meat to eat in winter. But if meat isn't prepared properly it may harbour the virus.

The health authorities in the Gorodische region claim they are doing daily checks on everyone in villages affected by the virus. They produce impressive statistics to back up the claim. But when I asked Mr Zagorov if he's being checked, he laughs. "What do you mean they're checking me? I took my own temperature."

Alexander Yurlov is an ornithologist with the Russian Academy of Sciences. Mr Yurlov's tests show that 10 per cent of the migrating waterfowl in the area carry some sort of virus. He says some birds will already have begun migrating west towards Europe. His advice to European governments is " check birds for viruses, and control places with a high density of waterfowl". As for keeping all poultry indoors, something the British Government says is not yet necessary, Mr Yurlov says it's a good idea. "It's better not to have contact between domestic and wild birds."

Damian Grammaticas is the BBC's Moscow Correspondent

QUESTIONS AND ANSWERS

What makes this differ from ordinary flu?

Every year the familiar flu virus, which has been kicking around for decades, undergoes a slight shift. This gives it a new lease of life and enables it to infect people who have built up immunity from previous illnesses. Some people die, usually the old and infirm. But most are not seriously affected because the change in the virus is relatively minor, allowing people to retain some protection. By contrast, this is a totally new virus to which no humans have any immunity. It could, therefore, kill tens of millions if it developed the ability to spread rapidly from human to human. When - and whether - it will do so is impossible to predict.

Could migrating birds bring it to Britain?

Almost certainly. The Government says that infected birds are likely to die before they reached our shores, but that is a false reassurance. It has been known for decades that waterfowl carry flu viruses without themselves being affected. Even if they did perish they would first pass it on to other birds that would carry it onwards. It has already been carried by migrating birds from China across Russia; there is no reason why it should stop there.

So it's coming from China?

Yes, from the massive Qinghai lake, high in the mountains in the far north-west of the country, a summer hub for migrating waterfowl from all over Asia. Thousands of birds died from the virus there this summer, and infected survivors have now begun their autumn migration to their wintering grounds. Ominously, there are unconfirmed reports that hundreds of people in this remote area have also died, and that some of the strains of the virus found in the birds have mutated so they can infect humans. China has denied this, and clamped down on information and research, leading to suspicions of a cover-up.

What can we do about it?

Pray - since whether, and when, the virus will mutate is largely outside our control. If it does, the one way to beat it is to develop a vaccine. But work on an effective one cannot begin until the mutation has taken place, and then it will take months to develop. In the meantime, the only hope is to slow down its spread. One way would be to keep poultry indoors, as the Netherlands is doing but which Britain has refused to do. Another is to rush anti-viral drugs, such as Tamiflu, to the source of any human outbreak. Roche, its manufacturer, has recently promised to give the World Health Organisation three million courses of the drug to do just that, but they will not all be ready until next summer.

Are other countries better prepared?

Yes, some are, despite the Government's claims to be leading the world. Australia is probably the best. It got in early in ordering supplies of Tamiflu, building up its stocks while supplies were available, and has plans to quarantine people suspected of carrying the virus, establish special "influenza hospitals" and set up special mobile medical teams to treat victims at home. Yet, even so, its Health Minister, Tony Abbott, says " a severe outbreak would test our national capacity in ways unknown for half a century". Japan and France were also quick off the mark. Last week the French President, Jacques Chirac, told his cabinet to spare no expense in readying the country, in marked contrast to the penny-pinching approach by ministers here.

Geoffrey Lean