But this innocuous-looking bug is wreaking havoc from Asia to Europe. And scientists say it is one of the most perplexing and powerful flu viruses they have ever seen.

It has jumped the species barrier from wild birds to domestic poultry; from domestic poultry to people; and from dead chickens to cats. This latter transmission is one of the creepier things about H5N1: Before this bug appeared on the scene, no one had ever seen a flu-riddled feline.

Whether this bug will cause the next pandemic is unknown. But researchers agree it’s a particularly menacing microbe, and they are watching it carefully.

Working in a high-containment laboratory at the University of Wisconsin-Madison, Stacey Schultz-Cherry and three other researchers have been studying the H5N1 virus.

When they work with the pathogen, they wear spacesuits, work in a sealed room with negative pressure so the air never leaves and are required to shower before leaving.

“There are numerous lines of protection,” she said.

Schultz-Cherry said the H5N1 virus seems especially adept at infecting and overwhelming cells in the lungs.

“The virus can actually grow much faster in lung cells (than other flu viruses),” said Schultz-Cherry, an assistant professor of medical microbiology and immunology.

It’s not clear why. It is possible the virus replicates too quickly in the lungs for the immune system to mount an attack, she said.

The kicker, however, is that it isn’t the work of the virus that swamps the lungs, but the victim’s own immune system. Once the virus has replicated and infected great swaths of lung tissue, the immune system kicks into overdrive, inflaming lung tissue and producing massive quantities of fluid. The result: pneumonia, suffocation and death.

Last week, a new study found that the H5N1 virus can trigger more than 10 times the inflammatory substances compared with those set off by the common flu virus, H1N1.

“The people who are dying are dying rather quickly,” Schultz-Cherry said. “The lungs are just destroyed.”

Researchers in her lab are working to develop anti-viral drugs as well as better diagnostic tests that can be used on viruses such as H5N1.

Some cases unrecognized?It’s not just the lungs H5N1 targets. The virus seems to have an affinity for other body parts, too, including the gastrointestinal tract and brain. Indeed, two Vietnamese victims never showed signs of respiratory infection, leading Oxford University researchers - in a 2005 New England Journal of Medicine paper - to wonder whether people were dying without ever being detected as flu victims.

In 2004, a 4-year-old boy was brought to the hospital after a severe bout with diarrhea. Soon after he was admitted, the boy started to convulse. Then he slipped into a coma and died. His brain was swollen. Postmortem analysis revealed H5N1 virus in his feces, blood, nose and the fluid around his brain.

His 9-year-old sister, who had died two weeks earlier, reportedly had similar symptoms, suggesting that she, too, succumbed to the flu.

Could people be dying of the flu without doctors knowing it? If the disease is not manifesting itself as it typically does - as an upper respiratory ailment - will doctors know to test for flu in a child with diarrhea, which is common in children living in Southeast Asia and China?

In another report, in the July 2005 issue of Emerging Infectious Diseases, researchers noted that the virus seemed to target the lower respiratory tract and the gut - not the upper respiratory tract, where typical human flus tend to hang out.

The researchers concluded that samples taken from the nose or throat of ailing patients - a common way to detect flu - might not show the disease.

According to Jeremy Farrar, one of the Oxford researchers who examined the case of the Vietnamese boy, the case underlines “the possibility that avian influenza can present itself in different ways.”

Speaking to the British Broadcasting Corp. in February, he added that while the “main focus has been on patients with respiratory illnesses,” it’s “clear that’s not the only thing we should be looking for. Therefore, the number of cases of H5N1 may have been underestimated.”

Indeed, because diarrhea and encephalitis are not at all uncommon in Southeast Asia, the disease may be felling victims incognito.

The cats that were infected - leopards and tigers at a zoo that were fed uncooked, and presumably infected, poultry - also showed these kinds of symptoms.

Tracing a deadly routeScientists know that roughly every generation or so, humans fall prey to flu pandemics.

The world has seen three pandemics in the last 87 years - the most recent being the relatively mild 1968 event - and 10 in the last 300 years.

“Pandemics happen,” said Mike Leavitt, secretary of the U.S. Department of Health and Human Services. “They’ve happened before, and they’ll happen again.”

Frederick Hayden, an epidemiologist at the University of Virginia and a flu adviser to the World Health Organization, said that a lot about H5N1 is still unknown.

He said there has been very little detailed autopsy information from those who have died from the flu, so it is too early to say what is typical - and not typical - of this flu.

If it’s true the disease can replicate in the gastrointestinal tract, as he indicated, the conventional wisdom of “It’s not a flu if you have stomach problems” may need to be updated.

But he and others say there is no doubt that this is a scary virus. Researchers who’ve watched the disease progress through chickens say it literally liquefies their crests and crowns.

H5N1 debuted in domestic poultry in 1996. Reports in Hong Kong, where it was first observed, documented its virulence: It killed a lot of chickens and ducks before jumping the biological fence to kill six people that following year. But within a few months it had disappeared - vanishing into the microbial ether whence it came. It remained hidden until 2003, when it re-emerged as bad as ever in East Asia.

To date, more than 120 people have been sickened by H5N1 - and 64 of these victims died. The vast majority presumably caught the flu as a consequence of handling, or being near, sick birds. But a handful of cases are believed to have been transmitted by other sick people.

These suspected human-to-human cases are extremely rare, however, allowing scientists a little room to breathe as they formulate a vaccine to vanquish the virus before it mutates into an efficient people-killer.

H5N1 has also been implicated in the deaths of millions of birds - some that died from the disease, others slaughtered for fear they had it.

Wild, dangerous, unpredictableSo what is it about this particular virus that makes it such an efficient and powerful killer? To understand that, you need to know some flu basics.

As with other living organisms, scientists have genetically classified the different strains of influenza virus. There are three major types, based on their protein composition: A, B and C. The C’s are very mild, B’s a little more potent. But it’s the last group that sends chills down the spines of infectious disease experts. The A’s can be wild, dangerous and totally unpredictable. The H5N1 avian flu is an A.

All flus have the same general appearance. Spherical in structure, their surfaces are dotted with three different kinds of proteins: hemagglutinin, neuraminidase and M2 proteins.

The hemagglutinin and neuraminidases are the H’s and N’s of flu nomenclature. The H’s help the flu to get into its victims’ cells. And the N’s enable it to get back out.

The M2s work as acid pumps - pumping the acid in or out as needed to break down cell and viral membranes.

Influenza, like all viruses, needs a host cell to survive. It breaks into a cell by using its H’s as keys: The gut cells in birds read these proteins as familiar, and allow them in. But in people, it’s usually the cells in the respiratory lining that allow entry.

This difference usually keeps flus from jumping species. But with just a little adaptive tweaking, a bird virus can change its H’s to accommodate human cells.

Once inside, the virus hijacks the cell and turns it into a makeshift breeding farm, where it replicates and sends off its clones - using the N’s to break out - to find new cells to restart the process.

Unlike most other living organisms, the flu uses RNA instead of DNA to encode it.

And in part, it’s because of this single-stranded code - instead of double as in DNA - that the flu, like other RNA viruses, is so ripe for adaptation and mutation.

A blip here, a beep there, can turn a benign flu into a raging killer. And these viruses certainly like to blip.

According to Mike Davis, author of “The Monster at Our Door,” unlike DNA, which “proofreads and corrects every copy of DNA” it makes, RNA strands “are careless hacks.”

As a result, error rates in influenza and other RNA viruses are very high, wrote Davis, adding that “each new strand of RNA is a mutant.”

This has led researchers to say it’s just a matter of time before H5N1 mutates into a people-phile - learning not only how to break into human cells but also then passing from person to person.

Becoming less deadlyThere is hope. Some researchers say despite the scary front H5N1 is showing, if it were to mutate to become easily swapped between people, it would probably lose some of its virulence.

“We’ve never seen a 50 percent human fatality rate” in a flu before, said Ira Longini, an epidemiologist and biostatistician at Emory University in Atlanta. And this one probably will not differ.

If there is any good news, it’s that the virus may become somewhat less deadly when and if it fully adapts to humans.

UW’s Schultz-Cherry said viruses can lose some virulence to gain some survivability. The theory is that if a virus is too deadly, it will kill off too many of its hosts.

“The viruses that are the most successful through time are the ones that have the lowest mortality,” she said.

A good example of that is the herpes virus, she said.

“It (H5N1) will probably have to give up some of its pathogenicity to ease its spread,” said Kelly Henrickson, an infectious disease expert at the Medical College of Wisconsin. “But that might mean it kills 20 percent of everyone” it infects.

Henrickson noted that the SARS virus had a mortality rate of only 9%, yet it had a major impact on the world economy.

SARS, or severe acute respiratory syndrome, emerged in southern China in November 2002, spread to Hong Kong four months later and eventually spread to 29 countries on five continents.

Because of quick efforts to control its spread, the disease infected only about 8,100 people, including about 800 who died.

One of the keys to containing the next flu pandemic will be surveillance and honest reporting of cases by countries in Asia, where it most likely will emerge.

Henrickson said he is concerned that autocratic governments in Asia may be tempted to cover up an outbreak. But because of the Internet and a high level of interaction among scientists around the world, it would very difficult to hide such an outbreak, he said.