Diseases & Conditions
Experts believe a flu pandemic is long-overdue. The most obvious source is bird flu.
Humans have been catching if from birds for the last decade, but as yet it has still not kick-started a global outbreak.
What is bird flu?
Like humans and other species, birds are susceptible to flu.
There are many types of bird, or avian, flu.
The most contagious strains, which are usually fatal in birds, are H5 and H7.
There are nine different types of H5. The nine all take different forms - some are highly pathogenic, while some are pretty harmless.
The type currently causing concern is the "highly pathogenic" Asian strain of the H5N1 virus.
Scientists have discovered four different subtypes of H5N1, and there could well be more. However, all are deadly to birds, and can cause disease - and death - in humans.
However, it is important to stress that H5N1 is overwhelmingly a disease that affects birds - and not humans.
It is true that humans have been infected, but almost all have been poultry workers who have come into intimate contact with birds. H5N1 cannot pass easily from human to human.
Migratory wildfowl, notably wild ducks, are natural carriers of the viruses, but are unlikely to actually develop an infection.
The risk is that they pass it on to domestic birds, who are much more susceptible to the virus.
How do humans catch bird flu?
Bird flu was thought only to infect birds until the first human cases were seen in Hong Kong in 1997.
Humans catch the disease through close contact with live infected birds.
Birds excrete the virus in their faeces, which dry and become pulverised, and are then inhaled.
Symptoms are similar to other types of flu - fever, malaise, sore throats and coughs. People can also develop conjunctivitis.
Researchers are now concerned because scientists studying a case in Vietnam found the virus can affect all parts of the body, not just the lungs.
This could mean that many illnesses, and even deaths, thought to have been caused by something else, may have been due to the bird flu virus.
Is it possible to stop bird flu coming into a country?
There is no failsafe way of preventing its spread.
Experts say proper poultry controls - such as preventing wild birds getting in to poultry houses - are vital.
In addition, they say monitoring of the migratory patterns of wild birds should provide early alerts of the arrival of infected flocks - meaning they could be targeted on arrival.
How many people have been affected?
As of January 2008, the World Health Organization (WHO) had confirmed 348 cases of H5N1 in humans in Azerbaijan, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Lao People's Democratic Republic, Myanmar, Nigeria, Pakistan, Thailand, Turkey and Vietnam, leading to 216 deaths.
For the latest WHO information on the numbers of humans infected and killed by avian flu, see related internet links section on right of page.
How quickly is the disease spreading?
After bird flu claimed its first human victim - a three-year-old boy in Hong Kong in May 1997 - the disease was not detected again until February 2003, when a father and son were diagnosed with H5N1, again in Hong Kong.
Since then it has spread westwards through Asia, the Middle East, Europe and Africa.
Despite mass culls, exclusion zones and other measures put in place to prevent its spread, the H5N1 virus has continued to travel.
In one week in February 2006, Italy, Greece, Bulgaria, Germany, Austria, France, Slovenia, India, Iran and Egypt confirmed their first cases of H5N1 in wild birds.
In April 2005, a dead swan in Scotland was found to have the strain.
But it can't yet be passed from person to person?
For the most part, humans have contracted the virus following very close contact with sick birds.
There may have been examples of human-to-human transmission, but so far not in the form which could fuel a pandemic.
A case in Thailand indicated the probable transmission of the virus from a girl who had the disease to her mother, who also died.
The girl's aunt, who was also infected, survived the virus.
UK virology expert Professor John Oxford said these cases indicated the basic virus could be passed between humans, and predicted similar small clusters of cases would be seen again.
It is not the only instance where it has been thought bird flu has been passed between humans.
In 2004, two sisters died in Vietnam after possibly contracting bird flu from their brother who had died from an unidentified respiratory illness.
In a similar case in Hong Kong in 1997, a doctor possibly caught the disease from a patient with the H5N1 virus - but it was never conclusively proved.
What would the consequences of a mass outbreak be?
If the virus gained the ability to pass easily between humans the results could be catastrophic.
Worldwide, experts predict anything between two million and 50 million deaths.
However the mortality rate - which presently stands at around 50% of confirmed cases - could decline as it mutates, they say.
Is there a vaccine?
There is not yet a definitive vaccine, but prototypes which offer protection against the H5N1 strain are being produced.
But antiviral drugs, such as Tamiflu which are already available and being stockpiled by countries such as the UK, may help limit symptoms and reduce the chances the disease will spread.
Concerns have been prompted by news that patients in Vietnam have become partially resistant to the Tamiflu, the drug that doctors plan to use to tackle a human bird flu outbreak.
Scientists say it may be helpful to have stocks of other drugs from the same family such as Relenza (zanamivir).
Can I continue to eat chicken?
Yes. Experts say avian flu is not a food-borne virus, so eating chicken is safe.
The only people thought to be at risk are those involved in the slaughter and preparation of meat that may be infected.
However, the Who recommends, to be absolutely safe all meat should be cooked to a temperature of at least 70C. Eggs should also be thoroughly cooked.
Professor Hugh Pennington of Aberdeen University underlined the negligible risk to consumers: "The virus is carried in the chicken's gut.
"A person would have to dry out the chicken meat and would have to sniff the carcass to be at any risk. But even then, it would be very hard to become infected."
What is being done to contain the virus in the countries affected?
Steps have been taken to try to stop the disease spreading among birds.
Millions of farmyard birds have been culled, while millions more have been vaccinated and confined indoors.
Areas where the disease has been found have been isolated and some countries have banned imports of live birds and poultry products.
In January 2006 international donors pledged $1.9bn (£1.1bn) in the fight against bird flu, while the World Health Organization has devised a rapid-response plan to detect and contain a global flu pandemic.
There are also measures recommended when a wild infected bird is found, including protection and surveillance zones.
However, experts do not recommend culling wild birds. Not only would this be a logistical nightmare, it is unnecessary. It is better to adopt a "wait and see" policy and cull infected farmyard birds if the disease is spreading among these poultry.