The bird flu virus that has arrived in Europe poses an unprecedented threat with experts warning yesterday that the number of human infections reported in Asia could be just the tip of the iceberg.
One of Britain's leading specialists in influenza said that a pandemic strain of bird flu that can spread between humans is more likely to arise as more birds become infected.
Sir John Skehel, a distinguished influenza expert with a lifetime in the field, said that in his experience there has not been a birdflu virus quite like the H5N1 strain affected south-east Asia, Turkey and Romania.
"This is an unusual situation. Since we've been recording there hasn't been the spread of a single virus as widely as the H5 virus," Sir John said.
The World Health Organisation said that the current outbreaks of bird flu in south-east Asia are the largest and most severe on record.
"Never before in the history of this disease have so many countries been simultaneously affected, resulting in the loss of so many birds," a WHO spokesman said.
In addition to the speed of its spread among birds, the H5N1 strain of bird flu can also infect humans and is unusually lethal, Sir John told journalists before a mission of British experts to China and Vietnam next week.
Thousand of birds - migratory species as well as domestic poultry - have been infected by the H5N1 strain of bird flu, which is also known to have infected at least 120 people, killing half of them.
So far there have been no reports of people in Turkey or Europe being infected by the latest outbreaks but Sir John warned that the risk to Europeans has increased "slightly".
Sir John, director of the National Institute for Medical Research in Mill Hill, where the influenza virus was first isolated in 1933, said that one of the great unanswered questions was how many people are really infected with the bird flu virus.
"How are people checking in the Far East for people who have recovered from the infection?" Sir John said.
"Are the total number of people who have been infected the tip of the iceberg and, beneath them, are there are a lot of others who have been infected? It is an important question," he said.
Sir John warned that trying to predict when and where the next flu pandemic will occur is a dangerous game.
"I think it is anybody's guess. The more it spreads, the more it is likely to infect... You just can't predict where the next one is coming from," he said.
The British delegation to China and Vietnam will investigate the measures being taken to monitor the spread of H5N1 and in particular the possibility that some people may be infected without developing serious enough symptoms to seek medical help.
They will spend 10 days looking at ways to improving collaboration with scientists in China as well as increasing the surveillance of potentially infected birds.
"We want to know how the Chinese are responding, how they are looking at their birds, how they are looking at their people, how they are set up and are there ways we can help," Sir John said.
Professor Colin Blakemore, chief executive of the Medical Research Council, said that announcing the decision to go to China and Vietnam was not a panic measure to the latest outbreaks in Europe because the trip was arranged many weeks ago.
"Vigilance and attention without panic is what we need. We've been planning this trip for some time," Professor Blakemore said.
Dr Alan Hay, director of the World Influenza Centre, said there was currently no vaccine against the bird virus and that it would take many months to develop one.
In a statement to MPs yesterday, health secretary Patricia Hewitt said £200m had been committed to fighting the pandemic and, as yet, there was no "direct threat" to the British population.
But she added: "We are of course planning for the very real risk of a very significant increase in the number of hospital admissions."