By Caroline Ryan
BBC News health reporter
H5N1 - a catchy little name currently on many people's lips.
There are fears the bird flu virus will mutate
It is the type of bird flu thought most likely to mutate into a pandemic strain - and fears of such a huge outbreak have led to the government updating its flu pandemic plans.
Outbreaks occur regularly - and we are overdue now.
That, as much as the fears over the spread of H5N1 among birds, is why preparations are being stepped up.
But the H5N1 flu "postcode" is not the only one to have caused humans problems over the last century.
And England's Chief Medical Officer, Sir Liam Donaldson says that while H5N1 will be the most likely culprit for creating a pandemic strain, it is not the only candidate.
"We can't just have tunnel vision and assume this is all going to happen from the current bird flu strain," he said.
Flu strains are labelled depending on their make-up.
In 1918, when 50m people died, H1N1 was the flu type. In 1957, H2N2 killed one million - and in 1968, during the last outbreak, H3N2 killed a million.
More recently, different strains have emerged in various parts of the world but have not developed into a pandemic.
In 1997, 18 people in Hong Kong were infected with H5N1, with six deaths.
In 2003, 84 people in the Netherlands were infected with the H7N7, leading to one death.
So far, H5N1 has killed more than 60 people in South-East Asia since 2003.
So strains have crossed the species without mutating into the most dangerous kind of flu, which spreads - and kills - easily.
Doctors and health planners also have to contend with seasonal flu each winter.
It infects 5 -10% of the population, compared to the estimated 25% who would be infected in a pandemic.
A pandemic would lead to a minimum of 50,000 deaths in the UK, experts warn.
But the UK sees thousands of deaths from seasonal flu every year.
In 1996-97, 21,400 people died from flu, while in 1999-2000 just under 22,000 were killed by the disease.
The numbers have come down. In 2004-2005, there were an estimated 1,268 deaths due to flu.
This is largely down to the success of the flu vaccination programme, Sir Liam said.
Each February scientists at the UK's National Institute for Biological Standards and Control sit down with experts from the World Health Organization and other centres around the world to decide which strains of flu that year's vaccine needs to protect against.
They then go back to the lab to develop a "safe" version of the virus which can be put into the vaccine.
This version looks like the disease-causing virus on the outside so the body's immune system can develop antibodies against it and fight the real thing when it appears.
But the replica version has a "safe" inside so it cannot make people ill.
Once these are ready in about April, manufacturers can begin making the vaccine, ready to give to people from October.
The same basic process will take place to develop a vaccine in a pandemic.
However, scientists and manufacturers will be under pressure to ensure it happens faster than the preparation of the annual vaccine.
And, because by its nature, a pandemic strain will be much more infectious than normal. Scientists will have to take care to ensure the version they send out to be put into the vaccine is safe, and cannot cause disease itself.
The UK government is tendering for "sleeping contracts", where companies would be on standby to produce 120m vaccine doses - enough for the whole population.
Two doses per person are thought to be needed, because in tests a basic H5N1 vaccine had to be given twice to be effective.
Sir Liam said the sleeping contracts would help the UK's pandemic response, because manufacturers would be secure in increasing their capacity in order to be ready.
And he said it would put the country to the "front of the queue" when the pandemic strain emerged and vaccine development could begin in earnest.
Sir Liam added: "We cannot prevent a flu pandemic, but we can reduce its impact."