Top public health experts are sounding a warning about the possibility of a bird flu pandemic. The dangerous avian flu virus has infected birds from East Asia to Europe. Human infections have been relatively rare so far -- 117 cases, according to the World Health Organization, all of whom appear to have gotten the disease from direct contact with birds.

Dr. Greg Poland of the Mayo Clinic says two of the three conditions are in place for a global flu epidemic, or pandemic. We already have a new virus for which we don't have immunity ... and the virus can sicken humans. "The remaining condition is for this virus to develop the ability to be easily transmitted from one human to another to another. When this happens, time will be described for those left living as 'before' and 'after' the pandemic."

Scientists warn that the virus could mutate, giving it the ability to be spread from person to person. At a recent briefing for Congressional staff on Capitol Hill, Dr. Tara O'Toole of the University of Pittsburgh stressed the urgency of the threat. "What we are talking about is not just another health issue. It is a nation-busting event."

A nation-busting event.

The best comparison to the potential threat is probably the 1918 flu pandemic, which killed some 20 million people worldwide. The death toll in the United States was about 675,000. Today, though, with triple the population, a similar virus might be expected to kill three times as many people. "But these numbers were for a virus that had a mortality rate of 1% or 2%," stressed Dr. O'Toole, "not the mortality rate of 40-50% that we're seeing with today's flu virus."

A global epidemic of deadly flu could bring national economies to a standstill. Dr. Constance Hanna, medical director at Honeywell Corporation, described a possible scenario:

"Imagine you're operating a business and 20-30% of your employees don't show up to work," said Dr. Hanna. "Others are not coming to work because they either have to stay home and care for sick family members or schools are closed, so children need caretakers at home to take care of children. Transportation systems are curtailed or shut down. And employees who may be trying to get to work can't get there because of transportation issues. And as a result of transportation issues, you can't ship product, nor can you receive critical components you need to build your products. In addition to that, critical infrastructure will or may fail -- food, water, power, gas, electricity."

Of course, unlike in 1918, today we have modern drugs and vaccines. But that may not save us, says Jeffrey Levi of George Washington University, if we don't have the money and organization behind it.

"It requires a major investment, however, to increase our production capacity, to increase the speed with which we can get vaccine into production, and finally to get those vaccines out into people's arms," cautioned Dr. Levi. "We also need antivirals. We're not necessarily going to get a vaccine up and running in time. We need an antiviral stockpile. We need it to be built. And we can't just stockpile them. We have to have a method of distributing them."

Dr. Levi says preparing for a flu pandemic in the U.S. alone could cost up to $8 billion. He also stressed the need to support the World Health Organization, which is keeping tabs on the spread of avian flu. And he urged creation of what news reports inevitably called a "bird flu czar." "We need to make sure that at the federal level we have clear leadership, government-wide," he continued. "That there's a single point person who's ultimately responsible and accountable for our pandemic preparedness."

In an hour and a half, the public health experts painted a pretty bleak picture of what could happen if the avian flu virus known as H5N1 mutates so that it can be passed from human to human. But Shelley Hearne, of the group Trust for America's Health, said the intention is not to induce fear. "This isn't about panic. This is about preparedness. Let's take this opportunity while we have a clear and present danger in front of us."

Meanwhile, the virus continues to spread. Officials in Turkey and Romania have confirmed the presence of the disease, apparently spread by migrating birds.