An insidious flu virus that has been tracked as a potential global killer has suddenly gone underground, and that´s got the world´s top flu experts worried.

Already the World Health Organization has put out a worldwide warning that the H5 influenza virus has transferred to ducks in Southeast Asia. What raises concern is that the ducks don´t seem to mind.

"We´ve found recently that the virus no longer kills ducks," says virologist Richard Webby. "Now it becomes a lot harder to get rid of it. There´s not necessarily great surveillance in that part of the world, so the best way to tell if a bird has the virus is when they fall over dead."

Webby is an assistant member in St. Jude´s Division of Virology, part of the department of Infectious Diseases. He works on behalf of the WHO with St. Jude virologist Richard Webster. They have tracked the H5 virus since it first appeared in Hong Kong in 1997. That was the year Webster ordered the slaughter of millions of infected chickens in order to protect other fowl.

Each time H5 has re-emerged in 2003 and again this year it has swept through huge populations of animals. On its own the virus produces a 70% mortality rate; the virus has infected many mammals and killed most of the 44 people who have contracted it.

Should the virus complete the mutation process and adapt to humans, Webster says it could be as virulent as Spanish flu which killed as many as 50 million in 1918, often within 24 hours of infection.

With the virus again in ducks, Webby says, it has a safe environment to go through the mutation process.

"The disease is asymptomatic because of a stable host-parasite relationship," Webby says. "This suggests that the granddaddy of the virus may have been in ducks. It went across to chickens and evolved into a chicken virus, and now has gone back to ducks."

This species leaping is troublesome because different animals carry different viruses in their digestive system. Farm animals that live in close contact with humans will also have viruses adapted to humans.

Flu virus has a distinctive tendency to take itself apart and reassemble, borrowing components from other viruses doing the same thing. Flu also produces thousands of mutations with these different bits and pieces.

The result, Webster says, are millions of different viruses, each looking for an opportunity to infect and flourish. Left to its own devices H5 could eventually turn into a global pandemic with the potential of killing hundreds of millions and making several billion people gravely ill.

"For this virus to start transmitting human-to-human it needs to pick up a gene segment from a human virus," Webby says. "Or it just goes through humans enough times; every time the virus replicates it introduces mistakes and mutations, and eventually one of those mutations becomes opportunistic."

St. Jude scientists discovered this process as they were developing ways to disassemble flu virus into its eight basic components.

That technique could be adapted to producing new flu vaccine in short order, tailored to the specific flu that is spreading at any one time: scientists could take apart the target virus and rebuild it so that it would confer immunity rather than sickness. The re-engineeed virus could be mass produced into vaccine in a matter of weeks using recombinant technology.

Flu virus today is grown in chicken eggs over a period of months, with a process that´s been common for 50 years. If there´s a bad batch -- as happened this year -- there´s also a vaccine shortage. Profit margins are wafer thin because the federal government is the single largest buyer. That´s reduced suppliers from a dozen 15 years ago to just two today.

Manufacturers are loathe to implement any new technology, such as recombinant culture, without higher potential profits.

It´s frustrating, Webster says, to be able to offer a new technology that improves quality, only to be turned down. He fears it may take a global outbreak of H5 to get governments and industry to pay attention.

With H5 becoming indolent in ducks -- all the while experimenting with mutations -- the only tool virologists have to stop it is a knife at the throat of infected animals: destroying all the mutated viruses by destroying all the carriers.

"Stopping this means eradicating the virus from the flocks that have contact with humans," Webby says. "We´ve done that successfully before in Hong Kong where we´ve succeeded by temporarily depopulating the flocks.

"It´s different now," he says. "We´re talking about animals in rural Vietnam, Cambodia and Thailand. These are small country flocks that are not regulated. And if an infected animal dies, there´s not a system to report it."