Within weeks or months, it will grow into a regional epidemic affecting more and more humans before jumping across international borders and becoming a pandemic that strikes tens of millions of people.
If you think this sounds like science fiction, you're wrong, say doctors. It happened three times in the 20th century, when flu pandemics struck in 1918, 1957, and 1968. The deadliest outbreak, in 1918, killed more than 20 million people.
And leading scientists believe the world could be ripe for another one. What makes them especially concerned are the recent outbreaks of bird flu across Asia.
The Asian bird-flu virus, known in scientific circles as H5N1, first appeared in humans eight years ago in Hong Kong. Of the 18 people who became infected in that case through close contact with chickens, six died.
The death toll was so high -- unlike for common human flu strains -- because humans have no natural immunity to H5N1 or other animal flus.
The Hong Kong authorities ordered the slaughter of every chicken in Hong Kong -- more than 1 million animals, to halt the spread of the virus. At first, they appeared to be successful. But in 2003, H5N1 resurfaced. Since then, it has spread to Vietnam, Korea, Thailand, Indonesia, and Malaysia as well as large swaths of mainland China.
More humans have become infected, as well as pigs. Albert Osterhaus, a virologist at the Erasmus Medical Center in Rotterdam, says this is an ominous sign.
"What we have seen in the last century is that we have had three major pandemic outbreaks of influenza. And all these outbreaks originated from the animal world," Osterhaus says. "There's an enormous quantity of influenza viruses in migratory birds. And these viruses are at the source of pandemic outbreaks by the fact that they are being transmitted from wild birds -- either directly or indirectly through poultry -- to pigs or humans. And if these viruses can indeed transmit efficiently from human to human eventually, then you have a virus that has not been in the human population before and therefore there is no immunity in the population at large. And this allows the virus to spread quite rapidly through the population."
Each time the virus enters a new species -- going from wild birds to poultry, to pigs or humans, it can mutate, adapting to its new environment.
The trigger for the pandemic, as Osterhaus notes, will be if and when the virus finds a way to pass easily between humans. The fact that H5N1 has been detected in pigs raises special concern. Pigs, which are prone to both bird and human flu strains, can act as a "mixing vessel" in the creation of new strains.
Osterhaus and several colleagues, writing in the latest issue of the respected British journal "Nature," are urging the world to prepare.
"We are not predicting that the H5N1 that is occurring at the moment in Southeast Asia will be at the basis of the next pandemic. But it could well be. It could be another avian virus," Osterhaus says. "We don't know for sure at the moment. But what's happening in Southeast Asia at the moment really shows us that we have to prepare ourselves adequately for the next pandemic outbreak of influenza."
The key, says Osterhauser, is having an effective, permanent global monitoring system to catalog the viruses that are already in the environment. Should one of those viruses mutate and begin spreading into the human population, scientists would be ready to respond quickly with a vaccine.
"It is virtually impossible to predict which virus exactly will be the next pandemic virus, what the virus will look like," Osterhauser says. "So what you can do is on the one hand -- if you have your monitoring system in place and know exactly which different viruses are circulating in wild birds, in domestic animals and can come to humans -- at least you can make a repository of those viruses and you could also then make seed viruses for those different strains."
The WHO -- mindful of the risk of a flu pandemic -- adopted a new set of international regulations this week that require the body's 192-member states to establish better monitoring systems to track public health threats. Under the new rules, countries will be required to increase their prevention measures for existing communicable diseases as well as to keep a look out for unusual patterns that could signal the emergence of new viruses. If any are detected, they will have to be reported to the WHO immediately.
Dr. Max Hardiman led the task force that developed the new rules. He tells RFE/RL that many countries in the postcommunist world face a dual task of rebuilding their old monitoring systems to track standard communicable diseases such as tuberculosis, cholera, or typhus, while refocusing on new threats.
"For most countries it's also a question of not just reestablishing the old systems, but actually making sure that there is a new focus on being able to detect the unusual and unexpected," Hardiman says. "It's one thing to have a surveillance system which gets regular reporting of recognize disease events. But a new focus is needed on picking up the unusual and the unexpected so that these are not overlooked because they don't have a disease label to attach to them."
The last time the WHO updated its rules was 1969 -- when fewer people traveled abroad and diseases thus took far longer to cross international borders. With the advent of cheap, ubiquitous air travel, the likelihood that a new flu strain or other disease will cross from one corner of the globe to the other in a matter of hours or days -- instead of weeks or months, is exceedingly high.