Health reporter, BBC News
Thousands of turkeys are being slaughtered in Suffolk in a bid to contain an outbreak of bird flu.
Does this mean the public should be worried?
What sets H5N1 apart from other avian viruses is the severity of the illness it produces in humans who are infected.
Scientists have also been surprised by its longevity, as many new infectious strains in bird populations are self-limiting and die out.
But Professor Neil Ferguson, professor of mathematical biology at Imperial College London, said considering how many millions of birds had been infected with H5N1 worldwide, cases of human infection had been incredibly rare.
He added: "The thing that really shocked us was that we were getting quite severe infections in humans. With other strains of avian influenza, people weren't affected so severely.
"H5N1 has been unusual in that it's been so successful in spreading around the world and that has surprised people."
However, for the virus to cause a human pandemic, it would have to be able to spread rapidly between people.
To date, there have been 270 human cases and 164 deaths - but only a few reports of human-to-human transmission.
"For an epidemic, every new person infected needs to cause at least one more infection and at the moment it's less than one in 20, probably even lower," said Professor Ferguson.
"Every new infected person represents a possibility that in that one person the virus will mutate and change or combine with another virus and then be able to transfer from person to person."
He stressed that humans would need very close contact with birds but also that there was a huge element of chance.
Dr Elspeth Garman, reader in molecular biophysics at the University of Oxford, said the longer we go without a pandemic the more unlikely it is that it will occur.
"It's been around for 10 years and I thought it would have mutated by now and it hasn't, even though the viral load is enormous.
"The important point is it's really, really hard to catch it from birds. It's got a huge mortality rate, around 61%, but it's very hard to catch."
She explained that the virus latched on to a specific receptor in the guts of birds to cause infection but this receptor is not present in the nose and throat of humans.
It is thought to be present in the lungs, or lower respiratory tract, which means people would have to be heavily exposed to the virus to catch it.
It also makes it very hard for the virus to be transmitted between people.
If a mutation occurred which enabled the virus to infect the nose and throat, or upper respiratory tract, it could then spread to others via coughing and sneezing, she said.
Although she believes this is unlikely.
Professor John Oxford, professor of virology at Queen Mary University of London, added: "A virus that's not really ready to spread might be able to get around in a family but to go further it would need two or three mutations."
However, he warned against becoming complacent just because the virus has been around a long time without causing a pandemic.
"In the end, I think we will get these mutations and we can thank our lucky stars there is a big barrier.
"Proper preparation is the key," he said, adding he had been immensely reassured by the quick response from the Department for Environment, Food and Rural Affairs (Defra) to the first large-scale outbreak among UK poultry.