The growing number of cases of avian influenza, or bird flu, in Asian countries is causing increasing concern.
But what is the disease and what are the possible risks to humans?
Q: How do humans catch avian flu?
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Avian 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 dries and becomes pulverised, and is 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 illness, and even deaths, thought to have been caused by something else, may have been due to the bird flu virus.
Q: How many people have been affected?
The World Health Organization said that, since December 2003, there had been 97 confirmed cases of avian flu and 53 deaths in Cambodia, Vietnam and Thailand.
Avian flu does have a high fatality rate. In comparison, Sars has killed around 800 people worldwide and infected at least 8,400 since it first emerged in November 2002.
Q: Can avian flu be passed from person to person?
There are indications that it can, although so far not in the feared mutated 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.
Q: Does this mean there is likely to be a large outbreak of bird flu?
Experts are concerned that this could happen. But in the Thai case, the virus was only been passed to close relatives and spread no further.
In addition, it had not combined with a form of human flu.
This is the real fear. Experts believe that the virus could exchange genes with a human flu virus if a person was simultaneously infected with both.
The more this double infection happens, the higher the chance a new virus could be created and be passed from person to person, they say.
Q: What would be the consequence if this did happen?
Once the virus gained the ability to pass easily between humans the results could be catastrophic.
Worldwide experts predict anything between 2m and 50m deaths and
the possibility of more than 50,000 additional deaths in the UK.
Q. Are there different strains of bird flu?
There are 15 different strains of the virus. It is the H5N1 strain which is infecting humans and causing high death rates.
Even within the H5N1 strain, variations are seen, and slightly different strains are being seen in the different countries affected in this outbreak.
These strains are also different to those seen in the past.
Pakistan has announced cases of the H-7 and H-9 in poultry, but no cases of these strains being passed to humans.
Q: Is there a vaccine?
There is not yet a definitive vaccine, but prototypes which offer protection against the H5N1 strain are being produced.
However antiviral drugs, which are already available, may help limit symptoms and reduce the chances the disease will spread.
Q: Can I continue to eat chicken?
Yes. Experts say avian flu is not a food-borne virus, so eating chicken is safe.
Professor Hugh Pennington of Aberdeen University told BBC News Online: "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."
Q: What is being done to contain the virus in the countries affected?
Millions of birds have been culled in an attempt to stop the spread of the disease among birds, which would in turn stop it being passed on to humans.
Experts say people in the UK are at "very low risk" of developing the disease at present.