In recent days, the world has taken far-reaching steps to brace itself against the swine flu: The World Health Organization has raised its pandemic alert level to 5 on a six-point scale, the United States secretary of Homeland Security has declared a public-health emergency, and federal officials have released a stockpile of vaccines to state and local public-health providers.
The US has recorded 109 confirmed cases and one death related to this new variant of the flu. Yet in a typical flu season, the Centers for Disease Control and Prevention (CDC) estimate, some 36,000 people die in America alone because of complications from the illness.
Given the relatively small footprint of the disease so far, why has the US and the world community responded so overwhelmingly?
The answer: caution in the face of what scientists and public-health officials say they don't know about the virus.
In situations like these, public-heath officials walk a fine line between taking what they see as prudent precautions and triggering what some specialists call excessive fear.
Year to year, scientists see small genetic variations in the flu virus, but not enough to raise doubts about individuals' immunity to them or the effectiveness of existing vaccines. But the new virus raises questions that scientists have not yet been able to answer fully.
"We're seeing a big change" in the virus's genetic makeup, says Theodore Cohen, a communicable-disease specialist at Brigham and Women's Hospital in Boston. Until these changes can be understood, the response is discretion.
Dr. Besser noted that reports are beginning to trickle in to the CDC of cases in which the individual involved is outside the cluster of cases so far where people had traveled to Mexico. That's where the outbreak originated.
Getting a clearer bead on the details of the agent and the evolution of the outbreak will allow public-health officials to tailor their responses more specifically, he said. Indeed, some biomedical researchers have taken an initial look at the agent's genetic makeup and infer that the outbreak could be relatively tame.
In addition, part of the challenge has to do with the evolving meaning of the term "pandemic," notes David Roepik, who heads Roepik & Associates, a Boston-based risk communication consulting firm.
Throughout much of the second half of the 20th century, pandemics came and went, but they often involved diseases for which vaccines or other medications were available, at least in the developed world.
But the outbreaks of SARS in 2003 and the on-and-off recurrence of certain strains of bird flu have changed the public-health community's perception about how it should respond to a potential pandemic. A 2006 avian outbreak involved only 115 reported cases, but 69 percent ended in fatalities. There were no effective medical treatments for either SARS or bird flu, and no general immunity among humans, Mr. Roepik notes.
The WHO's current warning level and the responses associated with it have their roots in these outbreaks.
"We are now thinking of 'pandemic' in those terms," says Roepik.
So far, he says, governments in the main have been doing an effective job in avoiding excessive fear. A few, however, have gone to extremes unsupported by existing scientific evidence. The Egyptian parliament on Wednesday reportedly ordered the death of every pig in the country, even though public-health officials say the problem is not related to the presence of pigs or consumption of pork products.
Even in the US, there have been "teachable moments," said Besser of the CDC. Vice President Joe Biden's advice Thursday that people not travel by airplanes or subways required a more nuanced clarification from Besser later in the day: "If you're ill, don't get on an airplane." For everyone else, "airplanes are safe; subways are safe. People should go out and live their lives."