Just as scientists are pursuing early-warning systems for everything from earthquakes to hurricanes, public-health officials are monitoring potential epidemics as never before.
There's just one problem. Better detection has spawned several false alarms over the past few decades, potentially undermining the credibility of warnings. That dynamic is pushing governments and news media into unfamiliar territory.
How should they warn about something that might never occur? Does talking about an uncertainty help - or simply raise unnecessary fears?
These questions are becoming ever more relevant as the world confronts the spread of the virus H5N1 - better known as avian flu, or bird flu. The challenge: Even if many scientists say it's only a matter of time before the epidemic spreads, no one can predict how soon the day will come, or whether it will prove as harmful to humans as it has to domestic and wild birds.
So what should they tell the public?
"We need an informed concern - that's different than fear," says Howard Markel, director of the Center for the History of Medicine at the University of Michigan and author of a book on global epidemics. The difficulty, he says, is "how not to play Chicken Little. Because if you keep saying the sky is falling, and the sky isn't falling, people don't listen to you after three years."
The world has had its share of scares that didn't pan out. Two previous avian flu scares in Asia in the 1990s resulted in only a handful of human deaths, not the thousands that some feared. In 2003, severe respiratory syndrome (SARS) earned weeks of headlines but only 774 fatalities were attributed to it - far fewer than the deaths attributed to communicable diseases in the world during a single day.
Perhaps the most famous false alarm came in 1976, when a soldier at Fort Dix in New Jersey fell ill and died from what was called "swine flu." President Gerald Ford authorized a $135 million immunization program. About 40 million Americans received shots before the program was shelved as unnecessary. No further deaths occurred, no swine flu epidemic developed, and eventually many questioned whether the government had overreacted badly.
Those who closely study the H5N1 virus, including scientists at the World Health Organization, say that their concern that it might become a pandemic among humans is real and based on sound medical research and theory.
Since the late 1990s, scientists have become increasingly alarmed as they linked H5N1 to epidemics among bird populations from Hong Kong to Thailand and, in rare instances, to fatalities among humans. What researchers fear is that the virus might mutate so that instead of jumping from, say, a chicken to a human, it would pass from person to person.
A study released in late June by the research group the Trust for America's Health estimates that a moderately severe outbreak of H5N1 in humans could kill more than 500,000 Americans.
Nothing like that has hit the world since 1918-19, when the spread of Spanish flu was blamed for some 500,000 deaths in the United States and 20 million or more worldwide. But that era was far different than today. Then the epidemic hit with little warning. Health officials coped as best they could. Now, public-health officials usually have plenty of warning.
Any effort to underplay a potential threat creates its own problems, experts say. "When you're not transparent, you're not forthright, it causes more problems than good," Dr. Markel says.
In 2003, for example, China hid the SARS epidemic from its people for months. The result was public panic - and international criticism that China's bureaucratic inaction caused the disease to spread further than it otherwise would have.
Sometimes, politicians can worry too much about public reaction, these experts say. "When you tell me about a new risk, very briefly I'm genuinely more frightened," says Peter Sandman, a risk-communication consultant in Princeton, N.J.
But only briefly. People may worry more about avian flu, he says, but they'll compensate by worrying less about something else, like pollution or crime.
"The notion that if we warn people about avian flu that they're going to stop being the levelheaded people they were yesterday is nutty," he says.
For example: The press doesn't shy away from reporting on hurricanes for fear that it will create hysteria, experts say.
Such lessons extend to reporting on epidemics, they add.
"The media needs to simply report what is. Give people information to let them make the judgment of how afraid to be and what to do about it," says David Ropeik, director of risk communication at the center for risk analysis at the Harvard School of Public Health and a former television reporter. .
"It's very hard to get media to do a story about a hazard that isn't happening yet," he says.
When Americans were told that there was a limited supply of flu vaccine last year, millions of Americans showed "some very decent behavior" by passing up shots so that those deemed to be at higher risk could receive them instead, Sandman says. Only a few behaved badly. "You level with people, and they do OK," he says.