Guan Yi, a microbiologist at the University of Hong Kong who has studied the virus since 1997, said it was prone to mutate in search of suitable hosts across different species.

"The virus is easily transmitted from one species to another. It can jump to many different species," he said in an interview.

Guan said the H5N1 virus carries a genotype that can be extremely unstable and generate many variants that are capable of jumping and latching onto different species.

"The H5N1 has the tendency to latch onto different species and invade multiple species," he explained.

"When it jumps from one species to another, it doesn磘 know if the new species likes it or not, so it becomes very unstable and generates many variants to allow the host to select the best one ... so as to increase its own chances of survival."

"It doesn磘 want to kill the host because that way, it kills itself too. So it has to change itself. It generates many variants that can even invade neighbours of the host," Guan said.

UNSTABLE, CHANGING, ADAPTABLE

Migratory waterfowl are reservoirs for the H5N1 and are not taken ill by it. But there is no vaccine to protect poultry, which is susceptible to the virulent virus, and it can wipe out entire flocks within 24 hours upon infection.

Last month, Dutch researchers reported that domestic cats can catch the virus, which means pets may spread the disease.

Governments around the world must have breathed a sigh of relief yesterday when Thai authorities said its declaration last week that a dog in the country had been infected with H5N1 was wrong, and probably caused by a mislabelled sample. However, the virus has been found in the past in pigs, which are susceptible to human flu viruses. The biggest fear of experts is that the H5N1 and a human flu virus will marry in swine and become easily transmissible among humans, setting off a pandemic.

So far, it has only been established that human victims of the virus were infected directly by contaminated birds.

Thailand last week reported its first probable case of human transmission of the virus, and Hong Kong experts suspect there might have been some cases of human-to-human transmission in 1997. But these have never been confirmed.

Guan called on governments around the region to address the problem at its root - by reducing human exposure to domestic flocks through tightly regulated farming, and shielding domestic flocks from migratory waterfowl.

"We need to stop the virus spreading among poultry and reduce the density of the virus in the environment. We have to cut down on the exposure of animals to this virus, only then can we stop it," he said.