The virus could spread to humans two different ways.

With luck, the world will escape the latest outbreak of bird flu with no more than the six human deaths already blamed on it and the loss of millions of chickens. But public health experts worry of a much greater disaster.

Their foreboding: A catastrophe they say is among the worst imaginable, a global outbreak of an entirely new form of human flu.

There is no clear sign that will happen. Nevertheless, avian influenza’s sudden sweep through Asia, along with its tendency for wholesale mutation, leave many wondering about the bug’s potential for rampant spread among humans. It is a possibility the medical journal The Lancet calls “massively frightening.”

“The question everybody is asking is, ‘Is this the progenitor to a pandemic?”‘ says Dr. Gregory Poland, chief of vaccine research at the Mayo Clinic.

Influenza pandemics typically strike three or four times a century. The worst in the past 100 years, the 1918-19 Spanish flu, caused an estimated 40 million to 50 million deaths. Another is considered inevitable and perhaps overdue, but when it will happen and how bad it will be are almost totally unpredictable.

The nightmare this time would be a flu virus leaping from birds to people and spreading, introducing a disease for which humans have no natural defense.

The potential source is the strain of bird flu that has moved rapidly through parts of Asia since December, infecting chickens in at least six countries.

Millions of birds have died of the flu or were destroyed by workers trying to contain the outbreak. The World Health Organization says eliminating this “animal reservoir” is urgent.

Avian flu is naturally carried by wild ducks, and it ordinarily does not attack creatures other than birds or pigs, so experts are especially concerned that this bird flu is occasionally infecting people.

Human cases have been reported in Vietnam and Thailand, including six deaths as of Saturday in Vietnam, the WHO said, and one suspected death in Thailand. Experts believe all caught the virus from chickens, not other people.

“We know there are two possible ways a new pandemic strain can emerge,” said Dr. Steve Ostroff, deputy director of the National Center for Infectious Diseases at the Centers for Disease Control and Prevention. One is a human flu virus that resurfaces after years of dormancy, so people have no defense built up from earlier bouts. The other is a non-human variety acquiring the ability to infect people and spread.

The latter may happen if somebody already infected with a human flu virus also catches the bird virus. Inside the body, these two may recombine into a new mutant, part-human virus, part-bird.

The more people are around infected chickens and other birds, experts say, the more chances there are for such a disaster to occur.

“If the virus continues to spread in chickens, it may adapt itself so it can grow in humans,” says Dr. Arnold Monto, a University of Michigan epidemiologist. “If it is transmitted human to human, then we are concerned this is the start of the great pandemic.”

The first time an avian flu virus was found to have infected people was in Hong Kong in 1997. At least 18 people fell severely ill and six died.

Experts believe a pandemic may have been averted that time by the rapid slaughter of Hong Kong’s entire poultry supply – an estimated 1.5 million birds killed in three days.

That flu virus was the H5N1 variety, one of 15 known subtypes of avian flu. The WHO calls it worrisome for several reason: It mutates rapidly and tends to acquire genes from flu viruses in other animal species; it is clearly dangerous to people; and it spreads quickly. Infected birds give off the virus for at least 10 days in their feces and oral secretions.

H5N1 appeared again last February, when two members of a family returning to Hong Kong from China became ill. One died and the other recovered. How and where they got infected was never learned.

The disease now circulating in Asia is the same H5N1, but it is so widespread that a quick purging, like Hong Kong accomplished seven years ago, is unlikely. Testing shows it has mutated but has not yet picked up any genes from human flu viruses.

If a bird flu pandemic occurs, could it be stopped? Many experts fear not. Flu is so contagious that quarantining victims, a method that eventually contained SARS last year, is unlikely to work.

Studies suggest that prescription drugs used to treat human flu strains could also keep people from catching the bird flu. However, spot shortages were reported during this winter’s U.S. flu outbreak, and supplies would quickly run out during a pandemic. No country has stockpiled the drugs, Tamiflu, Relenza and the older amantadine and rimantadine.

The WHO is already working on a prototype vaccine against the bird flu. But even the standard annual flu shot takes six months to manufacture, and experts doubt a new vaccine could be ready in time.

If there is evidence the bird flu is producing significant illness in humans, “there would be a full-bore effort to produce a vaccine,” says the CDC’s Ostroff. “It’s hard to predict the timeliness of it and how widely it could be put into people’s arms.”

EDITOR’S NOTE: Medical Editor Daniel Q. Haney is a special correspondent for The Associated Press.

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AP-ES-01-24-04 1314EST

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