Anti-viral drugs are being stockpiled, chickens are being brought inside,and yesterday vetinary experts from across the EU met to consider how they can avert a crisis. Nick Morrison looks at the virus which experts believe could claim millions of lives.

IT seems no longer to be a question of if, but of when. The spectre that has been lurking in the east has broken out of its enclave, and its arrival on our shores seems inevitable. Now, the only question is whether bird flu will prove as destructive as has been feared.

Since it was first identified in 1997, the strain of avian flu known as H5N1 has killed 57 people, but the virus has been confined to south-east Asia. Last month, it was identified as having spread to Russia and Kazakhstan. Migrating birds will ensure it is only a matter of time before it reaches the UK.

According to the experts, bird flu has the potential to cause the world's next pandemic, and could surpass even the Spanish flu outbreak of 1918 which claimed 40 million lives. The World Health Organisation (WHO) predicts anything from two million to 50 million deaths.

In response to this threat, the Dutch - among the world's biggest meat exporters - have ordered their entire flock of free range chickens to be kept indoors. Governments around the world are stockpiling supplies of anti-viral drugs and yesterday EU veterinary experts met to look at ways of restricting the spread of the virus. But what is bird flu? How does it spread? How dangerous is it? And what can be done to stop it?

Q What is bird flu?

A Avian flu was first identified more than 100 years ago, and is caused by a strain of the influenza virus, closely related to that which affects humans. Domestic birds, such as chickens or turkeys, are particularly susceptible to the virus, which triggers symptoms ranging from mild illness to death.

H5N1 is one of 15 strains of bird flu and was first spotted in Hong Kong in 1997, when it infected 18 people, six of whom died. To try and contain the disease, Hong Kong's entire poultry population of 1.5 million birds was slaughtered. The latest H5N1 variation first surfaced in Vietnam and Thailand in 2003, and has since spread to Cambodia, Indonesia, Laos, South Korea and Japan.

Q How is it spread?

A Avian flu normally affects just birds and pigs, but studies after the 1997 Hong Kong outbreak showed it jumped directly from birds to humans. Most of those infected worked closely with live infected birds. The virus is carried in the infected birds' guts and then excreted in their faeces. When the faeces dry and are pulverised, the virus is then inhaled. Infected birds which survive excrete the virus for at least ten days, helping it to spread at live poultry markets, and by migrating birds.

There are also indications that it can pass from human to human. In one case in Thailand, a girl is thought to have passed the disease to her mother. Both victims died, although an aunt who was also infected, survived. In another case, two sisters in Vietnam were thought to have died after contracting bird flu from their brother, who died from a respiratory illness.

Q Is it safe to eat chicken?

A Experts say the virus cannot be spread by eating meat from a bird which has been infected.

Q How does it affect humans?

A The symptoms are similar to other types of flu: fever, sore throat, coughs and, in some cases, conjunctivitis and severe respiratory illness. Scientists had previously thought the virus just affected the lungs, but studies now show it can affect all parts of the body, meaning that some deaths thought to have been caused by something else, could have been due to bird flu.

Q Why is H5N1 a particular concern?

A While other diseases have passed from animals to humans, notably ebola and Sars, they have not always combined a high infection rate with a high fatality rate. Ebola, thought to have crossed to humans from chimpanzees, kills around 80 per cent of those it infects, but is not easily transmissible from human to human. Sars has killed around 800 of the 8,400 people infected since the respiratory illness emerged in November 2002.

So far, 57 people have died after contracting H5N1, about half of those confirmed to have contracted the disease. But what is particularly worrying is the characteristics it shares with other flu viruses. The virus mutates rapidly and combines with viruses infecting other species. Although it is not easily transmissible between humans now, as more people are infected, there is more potential for the virus to mutate and combine with other viruses, making it more likely that a new mutation would emerge which has both a high fatality rate and is easily transmitted from human to human.

Q How likely is this?

A The rate of mutation means this is certainly a possibility, and this has happened with other influenza viruses in the past. But it is also possible that, in mutating to become more easily transmitted among humans, the virus could become less harmful.

But experts warn that we are overdue another pandemic. As well as the 1918 Spanish flu outbreak, there have been pandemics every ten to 15 years or so - in 1902, 1936, 1948, 1956, 1968 and 1977 - but the last was 28 years ago. If there is to be another pandemic, bird flu appears the most likely culprit.

Q Is there a vaccine?

A Although vaccines are being developed for H5N1, they will not be effective if the virus mutates into a form which is easily spread among humans.

Instead, it will take at least four months to develop a new vaccine in sufficient quantities.

More effective in the short-term will be anti-viral drugs, which are already available and may help limit the symptoms and reduce the chances of the disease spreading, although there is a danger that a mutated virus will become resistant to these drugs.

The WHO is planning to stockpile anti-viral drugs for use in poorer countries, and many countries, including the UK, are also building up reserves.

Q What can be done to stop it?

A One of the first measures is to prevent it from spreading. That is why the Dutch government this week ordered all the country's free-range poultry to be moved indoors, although Britain has decided against such a move at this stage.

Earlier this month, UK and US experts ran computer models looking at ways of halting the virus.

By identifying an outbreak at an early stage, and distributing anti-viral drugs to people nearest to those infected, the scientists suggested that it would be possible to contain its spread. But, they fear, if early containment fails, there will be little to stop the virus spreading across the world.

If preparations are not in place, the WHO has predicted that between two million and 50 million people will die, including 50,000 in the UK.

Even in the best-case scenario, the WHO estimates that between two and seven million people will die, and tens of millions more will need medical treatment.