Over the past few weeks we've collected your questions relating to bird flu and flu pandemic and asked our experts to answer them. Take a look at the most frequently asked questions:

1. How are the experts personally going to prepare for a pandemic? Mike Barnes
2. What can we do to prepare for a pandemic? Should we be stockpiling four months of food and water? Andrew Lam
3. What is the difference between a pandemic and an epidemic? Steve Bacon
4. How much protection does wearing a face mask give you against the virus? Richard Irving
5. If young, fit people are most at risk because of their healthy immune systems overreacting, would suppressing their immune system with drugs aid their chances of survival? Philip Davis
6. Will people who have had a recent seasonal flu vaccination have any extra immunity - no matter how small - to the H5N1 strain if it becomes pandemic? Tony Maclean
7. Don't all flu and cold viruses start off as bird viruses? If so, why is this one different? Martin Green
8. Is a lot of the fear around this generated because we are overdue a pandemic and thus looking for a possible cause rather than because this particular flu has something about it that indicates it will be the originator of a pandemic? Adam Ward
9. Will the virus be able to work its way through doors and windows? How close do you have to be to an infected person to become infected? Emma Wright
10. From what I understand H5N1 must mutate in order for it to become the pandemic virus. If this is the case, isn't it possible for the H5N1 to mutate into something less virulent or even perhaps completely benign? Mohiuddin Salam
11. Is air travel going to make a modern outbreak of pandemic flu much more severe than in 1918? Rob Hutchings

Will people who have had a recent seasonal flu vaccination have any extra immunity - no matter how small - to the H5N1 strain if it becomes pandemic? Tony Maclean

Current vaccines are made with H3N2 and H1N1 viruses. They will provide no immunity at all against H5N1. Experimental vaccines have been developed against H5N1 but there have been significant technical problems with them. In theory, a vaccine prepared with current strains of H5N1 would provide some benefit against a future pandemic H5N1 strain, but the pandemic strain will be different in subtle ways from the current one. Those subtle differences are referred to as mismatches. Vaccine mismatch against Fujian flu three seasons ago decreased the protective effect of the vaccine that year to only 38%.

Dr Jim Wilde, Medical College of Georgia Health System

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Robert Webster a foremost bird flu authority at the leading US centre at St Judes Children's Research Hospital recently told a conference in Memphis that 50% of a small group of laboratory mice injected with a component of seasonal flu vaccine survived exposure to a bird flu strain that ordinarily would have killed all of them. This is evidently because one of the three component virus types in current seasonal vaccine is H1N1 so there is an N match with H5N1. Other influenza experts were cautious of extrapolating from mice, and the N component is not the focus on standardising the vaccine H1N1.

Peter Dunnill, Professor of Biochemical Engineering at University College London

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Don't all flu and cold viruses start off as bird viruses? If so, why is this one different? Martin Green

There are many classes of virus that cause respiratory illness such as influenza (probably the worst), respiratory syncitial virus (almost as bad), parainfluenza virus, adenovirus, coronavirus (common cold) and rhinovirus (common cold). Of these, only the influenza virus has its origins in avian species. This particular H5 virus is different from other avian influenza viruses in being much more virulent; this is because it contains a specific genetic mutation that makes it so much more deadly.

Dr Jim Robertson, National Institute for Biological Standards and Control

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Is a lot of the fear around this generated because we are overdue a pandemic and thus looking for a possible cause rather than because this particular flu has something about it that indicates it will be the originator of a pandemic? Adam Ward

There is a certain amount of truth in this question. We are overdue a pandemic and there is no guarantee that the H5 bird flu will be the cause of the next pandemic; it could indeed be one of the many other subtypes of bird flu viruses. In order for the pandemic to start, the bird flu virus has to begin infecting humans and then acquire the ability to pass easily from human to human. Given the amount of H5 bird flu there is and that it has already started infecting humans, it is very good candidate.

Dr Jim Robertson, National Institute for Biological Standards and Control

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Keep in mind that there are numerous strains of influenza and that it is not necessarily the H5N1 strain that will lead to the next pandemic. However, why are we concerned about H5N1 specifically? Well, this virus has recently gained capacity to infect humans, which means it has jumped the species barrier from just circulating in water birds to infecting other species. This is unusual because influenza viruses are normally highly species-specific, meaning that viruses that infect an individual species (humans, certain species of birds, pigs, horses, and seals) stay "true" to that species, and only rarely spill over to cause infection in other species. The second reason the H5N1 virus is worrisome is because it has led to severe illness in a large proportion of known human cases. To date, 60% of the cases documented by the World Health Organization were fatal, and the majority of these people were ill for less than 10 days. This signals that the H5N1 virus is not only capable of infecting humans but also causing them serious disease.

Gina Samaan, Epidemiologist, World Health Organisation

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From what I understand H5N1 must mutate in order for it to become the pandemic virus. If this is the case, isn't it possible for the H5N1 to mutate into something less virulent or even perhaps completely benign? Mohiuddin Salam

In some circumstances, it has been observed that an animal virus that infects humans becomes less virulent as it passes from human to human - we call this attenuation - and certainly this might happen with the H5N1 virus if it mutates into a truly human form. There is, however, no guarantee that this will happen and it is highly unlikely that it will mutate to become completely benign.

Dr Jim Robertson, National Institute for Biological Standards and Control

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Your first statement is correct. For H5N1 to become the pandemic virus, it must gain the capacity to efficiently spread from person to person. This has not happened yet, but it is exactly the reason that the World Health Organization and other institutions are monitoring the situation closely.

To date, all outbreaks of the highly pathogenic form of avian influenza have been caused by viruses of the H5 and H7 subtypes. Highly pathogenic viruses possess a tell-tale genetic "trade mark" or signature – a distinctive set of basic amino acids – that distinguishes them from all other avian influenza viruses and is associated with their exceptional virulence. Not all virus strains of the H5 and H7 subtypes are highly pathogenic, but most are thought to have the potential to become so. Recent research has shown that H5 and H7 viruses of low pathogenicity can, after circulation for sometimes short periods in a poultry population, mutate into highly pathogenic viruses.

Regarding your question about whether the highly pathogenic H5N1 virus can mutate into a low pathogenic virus, the answer is that it is possible. Scientists cannot answer when or how such events take place, but the situation is monitored carefully by expert organisations and networks.

Gina Samaan, Epidemiologist, World Health Organisation

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Will the virus be able to work its way through doors and windows? How close do you have to be to an infected person to become infected? Emma Wright

Flu viruses are tiny (invisible to the naked eye, even under a standard microscope) and will certainly, in theory, be able to penetrate through gaps in doors and windows. It is, however, only in the vicinity of an infected person that there will be a sufficiently high enough concentration of the virus for it to infect someone else. This will occur through sneezing and coughing. It is difficult to put a precise figure on distance, but when someone sneezes, droplets could be spread out over a few metres. After a sneeze, the virus would eventually be diluted out in the air to levels at which there would not be enough to cause an infection, and also it will eventually die. The air will not 'fill up' with flu germs and you need not worry about leaky windows.

Dr Jim Robertson, National Institute for Biological Standards and Control

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Is air travel going to make a modern outbreak of pandemic flu much more severe than in 1918? Rob Hutchings

We don't expect a modern outbreak of pandemic flu to be worse than in 1918 because of air travel. In 1918, 99.5% of the population survived the outbreak, and of those who fell ill, 97-98% survived. These numbers are the worst case scenario we are expecting should pandemic flu hit us today.

If anything, a modern outbreak will be less severe than the 1918 pandemic, as our understanding of influenza is much better today, our hygiene levels have increased, we have antiviral drugs, vaccines, face masks etc. Even if we stopped all air travel, we wouldn't stop the virus - we'd just delay its arrival (and we'd certainly cripple the economy).

John Oxford, Professor of Virology at the University of London

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