More than 240 people have died from bird flu. The toll may not sound alarming but scientists fear that if the virus mutates into a human disease it could kill tens of millions within a few months.

Health experts agree a big pandemic of some sort is overdue and believe the strain of avian flu currently rampaging around the globe in birds is the most likely source.

The outbreak, the largest and most severe on record, originated in southeast Asia in 2003. It has now spread to around 60 countries and territories across the Middle East, Europe, Africa and other parts of Asia.

Even if there is no human pandemic, bird flu threatens the livelihoods of millions of people as health officials destroy flocks and other countries ban imports.

Disease and culls have already killed hundreds of millions of birds in dozens of countries.

Thailand lost its $1.2 billion poultry export industry to the European Union overnight.

Indonesia has seen over 110 human deaths from the virus, the highest in the world. China, Egypt, Thailand and Vietnam have also been hit badly.

Many of those who have died were under 25.

The World Health Organisation (WHO) says there have been a tiny handful of cases where an infected person has infected another but at the moment the virus cannot pass efficiently between people.

However, if it acquires the ability to spread the way normal flu does it could be devastating, both because it is so pathogenic and because humans do not possess any immunity to it.

No one can say when this might happen - it could be tomorrow or in a decade or indeed never. It is also possible the virus could lose its dangerous qualities as it mutates.

Health officials agree the virus is unpredictable, but there is little consensus on the possible risks to humans.

In June 2007, the head of Indonesia's commission on bird flu said tests had revealed the virus may have undergone a mutation in the country that allowed it to jump more easily from poultry to humans.

But WHO said there was no evidence that the virus was becoming more transmissible.

There were three major influenza pandemics in the last century - the 1918 flu, which killed between 20 and 100 million people, depending on the estimate, the 1957 flu, which killed about 2 million, and the 1968 flu, which killed about 1 million. All three mutated from forms of avian influenza.

H5N1 has similarities to the 1918 virus, suggesting it could affect healthy young adults more than the very old and very young who are usually most vulnerable to flu.

In 2006 the international community promised nearly $1.4 billion to help tackle avian flu and boost global surveillance. Donors pledged another $406 million in 2007.

For the moment, the risk of catching bird flu is minimal. Despite the infection of tens of millions of birds over large geographical areas, the number of humans who have contracted the disease is still in the low hundreds. See this WHO table for a country by country breakdown.

Close contact with sick or dead birds is the main source of infection.

The risk of catching the virus appears greatest during the slaughtering and preparation of infected birds for eating. In a few cases, children are thought to have caught the disease through exposure to chicken faeces while playing in an area used by poultry.

Symptoms in humans include fever, coughing and breathing problems. Pneumonia and multiple organ failure are common. Other symptoms can include diarrhoea, vomiting, abdominal or chest pain and bleeding from the nose and gums.

In poultry, H5N1 has been dubbed "chicken Ebola" because it causes massive internal bleeding. It can wipe out a flock within hours.

A human pandemic could start in two ways. If a person were infected with avian and human flu viruses at the same time they could swap genes creating a new strain transmissible between humans. In this case it would spread very rapidly.

Alternatively, the virus could adapt more gradually, improving its ability to bind to human cells during repeated human infection.

WHO has estimated that even a mild form of the disease would kill 2 million to 7.4 million people.

A serious pandemic would send the world into recession as workers became ill and others stayed at home to care for them or avoid infection. It would disrupt trade, transport and food supplies. Extra costs would include hospital and medical treatment and measures to contain the disease in birds.

The World Bank has estimated a pandemic could cost the global economy $800 billion a year, while the Lowy Institute, an Australian think-tank, says a really catastrophic scenario could kill more than 140 million people and cost $4.4 trillion.

Public health experts agree the world is nowhere near ready to cope with a human pandemic.

No vaccine is available because there is no way to predict what the pandemic strain will look like. It takes at least six months to make a new flu vaccine once the virus has appeared.

Many countries are stockpiling antiviral drugs like oseltamivir, commercially known as Tamiflu, which may improve survival prospects if taken within 48 hours of symptoms appearing.

Tamiflu and another drug, Relenza, known generically as zanamivir, were developed to treat seasonal flu and work by preventing the virus from replicating itself. But no one really knows how effective they would be against a bird flu pandemic.

Swiss drugmaker Roche, which manufactures Tamiflu, has received orders from scores of governments. The cost for developing countries is less than for developed countries.

"This is the first shot in history at trying to stop a pandemic at its outbreak," says David Reddy, Roche's pandemic taskforce leader.

The pharmaceutical giant is donating over 5 million treatments to WHO. Of these, 3 million will be stored centrally for use at the site of an outbreak and 2 million will be kept in regional stockpiles.

Roche has given companies in China and India the green light to manufacture the drug. Several other firms around the world have also started making copycat versions.

GlaxoSmithKline Plc is also to donate 50 million doses of its "pre-pandemic" bird flu vaccine to the global stockpile.

It said in 2007 it would deliver enough of the vaccine for 25 million people to WHO over a three-year period.

This may all sound reassuring, but countries need to stockpile enough to cover 25 to 50 percent of the population. And the costs are still going to put the drug out of reach of most people in the developing world.

WHO is under pressure from developing countries to devise a new formula for sharing samples of the virus, which is vital for plotting its evolution and developing vaccines.

Indonesia says it is unfair for foreign vaccine makers to use its samples, design vaccines, patent them and then sell the "discovery" back.

It withheld most of its samples in 2007. Along with other developing countries, it wants guarantees from richer nations and drugmakers that it will have access to cheap vaccines if it shares its samples.

Negotiations are set to continue after health officials failed to reach an agreement at talks in 2007.

Several countries, including Malaysia, have begun programmes to reach self-reliance in making vaccines and cut dependence on Western supplies.

Scientists believe the massive increase in Asia of human and bird populations living alongside one another has helped the disease to cross the species boundary.

The first human cases of H5N1 occurred in Hong Kong in 1997 when 18 people were infected and six died. Hong Kong's entire poultry population was slaughtered.

Avian flu reappeared in poultry in southeast Asia in 2003, affecting South Korea, Vietnam, Japan, Thailand, Cambodia, Laos, Indonesia, China and Malaysia.

In July 2005 the virus spread to Russia and Kazakhstan. Since then it has spread to Africa, other parts of Asia, the Middle East and Europe.

Human deaths have been reported in Azerbaijan, Cambodia, China, Egypt, Indonesia, Iraq, Laos, Nigeria, Pakistan, Thailand, Turkey and Vietnam. Non-fatal human cases have also been reported in Bangladesh, Djibouti and Myanmar.

There are fears that human cases could soon appear in India where at least 60 percent of the billion-plus population live in rural areas close to their livestock.

There are hundreds of strains of bird flu. Many wild birds carry flu viruses with no apparent signs of harm but other bird species including poultry develop disease when infected.

Migrating wildfowl are believed to be responsible for spreading the virus westwards, but trade in live poultry may have played a role in its movement around Asia.

WHO has warned that countries along migratory flight routes from central Asia may face a persistent risk of re-infection.

The arrival of bird flu in Nigeria in early 2006 raised serious alarm. Since then it has spread to other countries including Burkina Faso, Cameroon, Djibouti, Egypt, Ivory Coast and Niger.

The first known human fatality in sub-Saharan Africa was confirmed in February 2007 in Nigeria.

Millions of households live alongside chickens throughout the region, increasing the chances of the virus crossing into humans.

Poor medical, veterinary and laboratory services, lack of health education, porous borders and high mortality rates from other diseases mean a new human virus could easily spread undetected.

Farmers and villagers, many unaware of the risks, have been disposing of infected birds without protective masks and clothing.

But even with a concerted education campaign many are likely to continue selling or eating birds that have died because they cannot afford to throw away meat even if it might be infected.

Malnutrition and poverty will make people more vulnerable across Africa where impoverished health services are already struggling with the burden of AIDS, tuberculosis and malaria.