Health experts are warning a flu pandemic is imminent - with attention being focussed on the deadly H5N1 strain of bird flu which has emerged in South East Asia.
The elderly are likely to be particularly affected
The UK government has put together a plan for what it would do if a pandemic reached the UK.
Here, the BBC's Panorama programme looks at the dilemmas it poses.
The UK first began planning for a flu pandemic earlier this year, putting it ahead of the vast majority of countries in the world who have no plans in place at all for a pandemic.
But flu is not an easy thing to prepare for.
People will not have immunity to a pandemic flu because it will be a new type, and many will die. Others will spread it unknowingly.
The biggest unknown is how bad it is all going to be. The government is working with a range of outcomes from under 20,000 deaths up to 750,000.
Plans which might work to ameliorate suffering and panic at the lower end of this scale, will look completely inadequate at the other end.
The UK's plan will come into operation when the World Health Organization reports that a new variant of flu is transmitting from human to human.
How the UK acts will depend on how the virus behaves, what age groups it targets, where the flu comes from and how fast it transmits.
But the proposals lead to a number of dilemmas.
SHOULD THE GOVERNMENT SHUT SCHOOLS?
Schools, where large groups of people gather and mingle, are central to the spread of any infectious disease.
Children may be stopped from going to school in a flu outbreak
When children catch flu, the period of time during which they could pass it on to other people, is longer than for adults who have caught the same bug.
So it makes sense to shut schools to curb the spread of disease.
But that would mean parents having to be at home to look after the children - including nurses, doctors, police and local government officials, who would all be needed to cope with the pandemic.
It would also remove a large proportion of people from the workforce, affecting business.
And it's not clear that closing schools will actually stop children socialising with each other.
How long would parents be willing to quarantine their children for? If they are out on the street playing football with the neighbours then they may as well be in school.
Local education authorities would take the responsibility for deciding if schools should be closed, but a situation where one area chooses to shut schools while another keeps them open is probably not tenable.
Local authorities have asked for more guidance from the government.
It is a tight call, and for now the government has decided that making the decision will have to wait until a pandemic strain emerges.
If it is clear that it affects the young in particular, schools will be shut.
If not, if the virus kills the elderly as the seasonal flu tends to, they will probably remain open.
SHOULD THE GOVERNMENT RESTRICT AIR TRAVEL?
If, as is most likely, a pandemic starts in South East Asia, the way it will arrive in Britain is on an aeroplane, carried by a human.
Given this knowledge, it would seem to make sense simply to stop flights coming into the country.
But if air travel was stopped as soon as a pandemic was identified, thousands of UK citizens would be stranded abroad.
During the Sars outbreak, travellers in the affected area were checked
Even shutting routes into the UK from one country in South East Asia would result in abandoning travellers, business people and diplomatic staff to the flu.
It would also be hugely detrimental to the affected area, potentially stopping the flow of aid and medical supplies.
And if you cannot stop 100% of flights, it's not really worth stopping any.
Modelling work has shown that if you even let in 1% of normal flights you have merely slowed the spread of flu by a few days or weeks, at vast cost to the economy.
Australia has already stated in its pandemic plan that entry screening will be imposed during a pandemic.
However the UK government does not currently believe that this will be effective.
The main problem is that diagnosing flu is not easy: people can have and spread flu, before showing any symptoms.
Some might develop symptoms during the flight, while others may carry the flu without ever showing symptoms.
Potentially you could qurantine people in the same way that live animals are quarantined today but this is an extreme measure which may not be popular, or possible.
There is an acceptance amongst the government's scientific advisers, that come a pandemic ministers may respond to public demands to screen passengers at airports - even though the effectiveness of this measure is not yet proven.
If other countries, like Australia, enact this measure then there will be pressure on British politicians to do the same.
Anti-virals are drugs which can be used to treat flu, and are potentially life saving.
The government has currently ordered enough Tamiflu to treat 25% of the population from its manufacturer Roche.
The government is stockpiling millions of courses of the drug
The stockpile of 14.6m doses will be in place by September 2006.
When a pandemic strain emerges, doctors will see which groups are most at risk, and allocate the stockpile accordingly.
The elderly, and people with weakened immune systems are likely to be among those at the top of the list.
People need to be given the drug within 48 hours of developing flu symptoms for it to be effective.
How the NHS gets the drug to people in this timeframe is still being worked out.
It may be that there are 'flu centres' where you go to pick up the drug or that it is actually delivered to your door by a health professional.
Tamiflu can also be given to prevent people getting flu - but this only works if they are taking the drug when they are exposed to the virus.
So using the drug this way would mean a stockpile would be reduced very quickly.
But it may be useful to use Tamiflu as a preventive drug in some cases in a bid to stop the spread of the virus.
However, using the drug in this way would mean obtaining more of it, and demand for Tamiflu greatly exceeds supply.
In addition, these drugs have not been tested in a pandemic situation, so no one is certain they will work.
If the human to human transmission of flu was spotted quickly, anti-virals could be given at the point of outbreak in a bid to wipe the virus out by giving it no human hosts to spread to.
Roche has offered to provide the World Health Organization with a stockpile of 3m doses of the drug which would be shipped to the scene of the outbreak when it was discovered.
But come a pandemic, the biggest stockpiles will be held by governments like our own.
Would it be better to send this stockpile to the country where the pandemic occurs, or keep it and wait for the flu to come?
Again this is not an easy call. If the containment strategy doesn't work then the government would have used up part of a stockpile which could have been used for treating sick people at home.
If a pandemic were to start closer to home in say Romania, Turkey or another European country, the decision is likely to be harder.
We might feel more responsibility to closer neighbours, and our economic ties with these countries are stronger.
Even some of our essential services, such as domestic gas, are dependent on European suppliers.
Across Europe, stockpiles of anti-virals are nowhere near WHO recommended levels.
The EU, as part of its co-ordination of pandemic plans - which will be discussed later this month, is considering creating its own 'solidarity fund' in order to pay for anti-virals and other measures if a relatively unprepared country is affected first.
Perhaps the most contentious questions of all concern vaccines.
A vaccine against the pandemic strain cannot be made until it appears
No vaccine would be available until four to six months into a pandemic - because scientists have to see the flu strain before they can develop a vaccine.
People will then have to wait their turn for immunisation. Priority groups like healthcare workers will be vaccinated first.
But the government is building up a contingency stockpile of 2m-3m doses of H5N1 vaccine which can be manufactured now.
This is not yet a pandemic strain. The vaccine might offer some protection against the strain which does emerge - but nobody can be certain about this in advance.
The UK government is also tendering for a "sleeping contract" so that a vaccine manufacturer can be primed to produce enough vaccine for the UK population - 120m doses - as soon as the pandemic strain is identified.
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