If both viruses wind up in the same individual, they could mix genes and mutate into a form that is both very pathogenic and easily transmitted among humans, said John Oxford, a virologist at London Queen Mary's School of Medicine and Dentistry.

"We don't want a situation where you have a virus with the spreadibility of the pig virus with an H5 stuck on it. That is something we worry about, to put it mildly," he said in a phone interview.

On an influenza "Richter scale" of concern, if an ordinary seasonal flu rates a level three and the swine flu a level five, then bird flu on its own would be a six and "an H5N1 virus swapping genes with a pig virus would be the highest of all, at least a seven," Oxford said.

Since 2003, the H5N1 strain of bird flu has killed more than 250 people in a dozen countries, according to the World Health Organisation (WHO).

More than 80 percent of the deaths were in East and Southeast Asia, with 115 in Indonesia, 56 in Vietnam, 25 in China and 17 in Thailand. There have also been 23 fatal cases in Egypt, the most recent two weeks ago.

But while the strain has killed almost 60 percent of those infected, it does not spread easily among people -- virtually all its victims had extensive, direct contact with poultry.

By contrast, the swine flu that erupted in Mexico earlier this year has moved across the globe much more quickly, but thus far has not proved especially lethal.

All told, the WHO has verified some 1,900 cases in 23 countries. Mexican officials report 42 deaths, with two more registered in the United States.

The nightmare scenario for virologists is a mutated virus combining high infection potential and virulence.

"The danger of a genetic recombination is real, in animals and in humans -- this is something we fear," said Pierre Duplessis, Special Envoy for pandemic influenza at the International Federation of Red Cross and Red Crescent Societies.

"We know that in the current genome of the (swine) flu, there is a sequence that comes from birds, so it is possible that the virus could take on more aggressive attributes and become more virulent," he told AFP.

For Nigel Dimmock, an emeritus professor at University of Warwick in Britain, the risk is real but slight.

"There is a concern, but the formation of these viruses as viable human pathogens is very difficult for the virus to achieve," he said by phone.

Reassortment, or the mixing of genetic material, occurs when two flu viruses -- each with eight segments -- get into the same host cell, he explained.

But even if the chances of that happening remain low, health officials should remain vigilant, Oxford insisted.

"If the pig virus is going to move through a region with millions of people, these million-to-one chances sometimes come to fruition," he said. "The current swine flu could be a kind of dress rehearsal for a bigger outbreak of H5."

Duplessis and other experts caution that it is far too early to "declare victory" against the swine flu, even though its spread appears to have slowed.

All three of the great pandemics of the 20th century, they point out, began with mild "herald waves" in the spring that were followed by far more deadly attacks later in the year.

Mounting criticism that health authorities have overreacted -- shutting down schools and transport system, quarantining foreigners, closing travel routes -- may be misplaced, suggested Oxford.

"The countries of most concern are those where H5 is killing people -- Egypt, Indonesia, China, Vietnam -- and these are among the countries making gigantic efforts to stop" the swine flu, he said.

Didier Houssin, France's top health official for bird flu, also cautioned last week that "we must not forget that the H5N1 virus remains very active".

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