GENEVA -- For years after its appearance in 1997 in Hong Kong, the current bird flu virus seemed corralled in a few east Asian countries. But in the past four months, it has spread across Europe and into Africa, bringing to 31 the number of countries with sick birds.
People have caught it in a quarter of those, and just six people outside east Asia have died. The virus is still not easily caught by humans.
Even so, its sudden sweep across continents on the wings of birds has stunned public health officials. And most say they cannot predict where or when this disturbing germ might mutate into a form that could unleash a deadly flu epidemic.
"Anywhere the virus lands," said Dr. Mike Perdue of the global influenza program for the World Health Organization.
For many months, most experts said Asia was the most likely starting point because of its large population and ubiquitous animal markets. And many still believe that. But it's all speculation.
"It could be Asia. It could be somewhere else," said Maria Cheng, a spokeswoman for the World Health Organization.
"Turkey would have been logically a place that you wouldn't want to see it happen because we saw many cases in a short period of time," said Perdue, referring to the frightening spurt of human cases and the deaths of four children in January.
Dr. Scott Dowell of the U.S. Centers for Disease Control and Prevention said it's difficult to predict the spread of the disease in the next few months, because public health officials have little experience tracking a disease spread by migratory birds.
"I'm not an expert on migratory birds carrying human pathogens. I don't know who is. We're going beyond our experience," said Dowell who heads CDC's global disease detection program.
The closest analogy he could think of is West Nile virus, a bird-associated illness that began in Africa and landed on the East Coast of the United States in 1999. Within four years, it had spread to the West Coast.
As to where the virus has the best chance of morphing into a more serious human threat, Nigeria, where many people live close to their chickens, poses a particular threat. Its 800 federal veterinarians lack the support they need to combat the disease, said Dr. Peter Cowen, former director of a WHO coordinating center in veterinary public health.
"What's missing there is the laboratory infrastructure, the field infrastructure that would allow them to get out in the field to get samples, simple things like enough vehicles to get around," he said.
Cowen, an associate professor at North Carolina State University in the United States, believes it may be time to change strategies: Abandon efforts to quarantine and kill poultry flocks and instead focus on vaccinating them.
"We should be giving that some very serious thought," he said.
Currently, each country decides what to do.
"We don't really have a truly global infrastructure to fight this potential pandemic," he said. "When an epidemic moves into a new territory, it means a new member territory asks help from WHO rather than having a centrally coordinated disease control strategy" that all countries follow, he said.
Many governments aren't adequately paying farmers to destroy their fowl, and "you get sick birds in the market," Cowen said.
At the center of concern is the current strain of the bird flu virus -- known as H5N1, which has set records for its spread and deadliness among birds -- both domestic poultry and wild fowl. About 180 million birds have been killed by the disease or slaughtered in attempts to control it, according to the United Nations' Food and Agriculture Organization.
It has remained a relatively difficult disease for humans to catch. In the initial 1997 outbreak in Hong Kong, 18 people caught it and six died, WHO says. Hong Kong's entire poultry population of about 1.5 million birds was slaughtered in three days to control the outbreak.
H5N1 re-emerged in Hong Kong in February 2003. Since then WHO has confirmed the virus infected 170 people and that 92 of them -- more than 50 percent -- died. Until January the human cases of the disease were restricted to Asia -- Cambodia, China, Indonesia, Thailand, Vietnam. Last summer it was found in birds in Russia and Kazakhstan on its way to Europe. This year the first human cases were confirmed in Turkey and Iraq.
Experts see two likely ways for the virus to change into a strain that could set off a human flu pandemic -- either by mutating so that it becomes contagious among people or by combining with a regular human flu virus in a person infected with both at the same time.
Even though the spread of the disease in birds has been dented, WHO is holding its pandemic alert level at 3 on a scale of 6, where it has been since the virus first emerged in Hong Kong eight years ago.
That means very limited human-to-human transmission of the disease or none at all. Evidence of increased person-to-person spread would move the alert level to 4; significant transmission, level 5; and the final stage, "efficient and sustained" transmission, level 6.
A frustration in preparing for a pandemic is that no sure-fire vaccine can be made until the virus changes into a strain that easily infect people. And it will takes months to produce.
To shorten the time needed, researchers are already preparing a vaccine based on the current H5N1 virus with the intention of substituting genetic material from the pandemic virus should one emerge. Manufacturers would then be able to produce usable quantities in six months.
WHO is urging people in the meantime to be vaccinated with regular flu vaccine and governments are advised to stockpile antiviral medicine, mainly Tamiflu, because that has been somewhat effective at combating the virus.
Dowell of the CDC said public health officials have been establishing systems to quickly contain any potential future human outbreaks. He cited Thailand as making terrific strides in establishing surveillance systems that look for human cases, a network of labs that can quickly confirm bird flu in people, and rapid response teams that can restrict travel in outbreak areas and track down human contacts of ill people.
But Thailand has been working on its response system for two years, said Dowell, who previously was head of the CDC's Thailand-based international emerging infections program.
"The countries hit in the last month are going to have to start the process, and some of the countries have a lot less resources," particularly those in Africa, he said.
But Perdue of the WHO said countries have done surprisingly well, including Iraq, which has moved quickly to contain chicken outbreaks and the two confirmed human cases. India, Nigeria and Egypt also have responded well.
"Everyone pulls out their best efforts to cover this situation," he said.
Controlling the disease in birds is the top priority, said Perdue.
"Our best approach is just to do everything we can to keep it out of people," Perdue said.
Associated Press writer Mike Stobbe in Atlanta and medical writer Marilynn Marchione in Milwaukee contributed to this report.