From outbreak to overreaction?

So is this new swine-flu outbreak the next great plague or a global spasm of paranoia?

Are we seeing a pandemic or a panic?

The pathogen that has seized the world's attention has an official name (swine-origin influenza A H1N1), an acronym (S-OIV), a nickname (swine flu) and an apparent birthplace (Mexico). But the essential nature of the pathogen, its personality, its virulence, remain matters of frenetic investigation. Like all influenza viruses, it is mutating capriciously, and thus is not a static and predictable public-health threat but an evolving one.

The bug has gone global, showing up in Asia on Friday with the first reported case in Hong Kong. It also popped up in Denmark and in at least six more U.S. states.

But there has been some flu-scare backlash, with some officials questioning whether schools are too quick to close their doors at the first hint of the virus.

The World Health Organization (WHO) addressed the pandemic-versus-panic issue Friday by cautioning the public against leaping to any conclusions about the virulence of the virus. It has yet to show lethality outside Mexico (the one person to die in the United States was a toddler who traveled from Mexico to Texas), though that doesn't mean it will remain a mild pathogen in the weeks and months to come, officials said.

Influenza is a simple virus, with eight genes, but it makes poor copies of itself, leading to constant mutation. Most of those mutations are dead ends, but, given enough chances, the virus can become more infectious or more lethal. Although the United States is past its flu season, the Southern Hemisphere is just starting its season, entering the cold months when influenza can become explosive.

Some positive news surfaced Friday: Mexican scientists said the contagiousness of the swine flu is no greater than that of the seasonal flu that circulates every year. And a preliminary genetic analysis hasn't turned up any of the markers that scientists associate with the virulence of the 1918 Spanish influenza virus, said Nancy Cox, head of the flu lab of the Centers for Disease Control and Prevention (CDC).

In New York City, which has the most confirmed swine-flu cases in the United States with 50, the illness has not spread far beyond cases linked to one Roman Catholic school. In Mexico, the epicenter of the outbreak, very few relatives of flu patients seem to have caught it.

Almost everyone who became ill from the virus is either recovering or already well.

Still, it was too soon to be certain what the virus will do. Experts said the only wise course is to prepare for the worst.

The 1918-19 pandemic has cast a long shadow over the current health emergency. That virus circled the world, eventually infecting nearly everyone and killing at least 50 million people.

Pandemic lite?

Jeffery Taubenberger, the National Institutes of Health researcher who reconstructed the 1918 influenza virus, said he is growing the new swine-flu virus — H1N1 — in his lab.

"We're very early on in figuring out what makes this virus tick. I am loath to make predictions about what an influenza virus that mutates so rapidly will do," he said. But he thinks it will spread across the planet: "My prediction is that this strain will continue to spread, and it is very likely to become a pandemic virus, if it's not already a pandemic now. That does not mean that this has to be a very severe pandemic like 1918."

Michael Osterholm, an epidemiologist at the University of Minnesota, said the situation is analogous to forecasting a hurricane when meteorologists know only that there is a high-low pressure gradient in the Atlantic. "Anyone who gives you an answer right now, do not listen to them about anything ... ," Osterholm said.

WHO spokesman Gregory Hartl noted Friday that the public may misunderstand the word "pandemic." The term refers to where an illness spreads, not its severity.

A major unknown is the swine-flu virus' "case-fatality rate": the fraction of infected people who die. For the Spanish influenza, it was 2 to 2.5 percent for the U.S. as a whole, but in military camps and on troop ships, the rate was a brutal 7 to 10 percent, and in some Inuit villages it soared to 70 percent.

The other two flu pandemics of the 20th century were far milder. The Asian influenza of 1957-58 had a fatality rate of 0.2-0.5, and the rate during the Hong Kong flu of 1968-69 was lower, about 0.1 percent, close to what it is for seasonal flu.

In a typical U.S. seasonal flu season, about 200,000 people are hospitalized and 36,000 people die.

The case-fatality rate of the swine flu will become certain only when epidemiologists can track its behavior from the moment it arrives in a population, a difficult task under the best circumstances, which the current circumstances in Mexico aren't.

The question is how many other people contracted influenza but never got very sick. Researchers must draw blood from a sample of people in affected towns and cities to estimate how many people were infected and never knew it.

Early signs favorable

The early signs from the U.S. and a few European countries where the strain is spreading suggest it is not unusually dangerous, as there have been few deaths. If that continues to be true, it may help explain the mysteriously high mortality in Mexico. It may be that Mexico already has had hundreds of thousands, possibly millions of cases, all but the most serious hidden in the "noise" of background illness in a crowded population.

Mexican scientists also reported Friday that the virus' ability to spread from person to person is "fairly low" and that it's no more infectious than normal seasonal flu.

Material from The Associated Press is included in this report.