It could kill a billion people worldwide, make ghost towns out of parts of major cities, and there is not enough medicine to fight it. It is called the avian flu.
This week, the U.S. government agreed to stockpile $100 million worth of a still-experimental vaccine, while at the United Nations Summit in New York, both the head of the U.N. World Health Organization and President Bush warned of the virus' deadly potential.
"We must also remain on the offensive against new threats to public health, such as the Avian influenza," Bush said in his speech to world leaders. "If left unchallenged, the virus could become the first pandemic of the 21st century."
According to Dr. Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University's Mailman School of Public Health, Bush's call to remain on the offensive has come too late.
"If we had a significant worldwide epidemic of this particular avian flu, the H5N1 virus, and it hit the United States and the world, because it would be everywhere at once, I think we would see outcomes that would be virtually impossible to imagine," he warns.
Already, officials in London are quietly looking for extra morgue space to house the victims of the H5N1 virus, a never-before-seen strain of flu. Scientists say this virus could pose a far greater threat than smallpox, AIDS or anthrax.
"Right now in human beings, it kills 55 percent of the people it infects," says Laurie Garrett, a senior fellow on global health policy at the Council on Foreign Relations. "That makes it the most lethal flu we know of that has ever been on planet Earth affecting human beings."
The Council on Foreign Relations devoted its most recent issue of the prestigious journal, Foreign Affairs, to what it called the coming global epidemic, a pandemic.
"Each year different flus come, but your immune system says, 'Ah, I've seen that guy before. No problem. Crank out some antibodies, and I might not feel great for a couple of days, but I'll recover,'" Garrett says. "Now what's scaring us is that this constellation of H number 5 and N number 1, to our knowledge, has never in history been in our species. So absolutely nobody watching this has any natural immunity to this form of flu."
Like most flu viruses, this form started in wild birds -- such as geese, ducks and swans -- in Asia.
"They die of a pneumonia, just like people," says William Karesh, the lead veterinarian for the Wildlife Conservation Society. "When you open them up, you do a post-mortem exam. Their lungs are just full of fluid and full of blood."
Karesh has been tracking this flu strain for the last several years as it has gained strength, spreading from wild birds to chickens to humans.
"We start at a market somewhere in Guangdong Province in China," explains Karesh. "And it's packed with cages, and you'll have chickens, and you'll have ducks. You might have some other animals -- cats, dogs, turtles, snakes -- and they're all stacked in cages, and they're all spreading their germs to each other."
In response, Asian governments have killed millions of chickens in futile attempts to stop the flu's spread to humans.
"The tipping point, the place where it becomes something of an immediate concern, is where that virus changes, we call it mutates, to something that is able to go from human to human," says Redlener, director of the National Center for Disaster Preparedness.
Scientists in Asia and around the world are now working around the clock as they wait for that tipping point.
"Unlike the normal human flu, where the virus is predominantly in the upper respiratory tract so you get a runny nose, sore throat, the H5N1 virus seems to go directly deep into the lungs so it goes down into the lung tissue and causes severe pneumonia," says Dr. Malik Peiris, the scientist who first discovered the so-called SARS virus, which killed 700 people and drew worldwide attention.
To date, there have been 57 confirmed human deaths, and another suspected one last week in Indonesia. Scientists say the humans have only been infected by birds. However, they add, every infected person represents one step closer to the tipping point.
"Once that virus is capable of not needing the birds to infect humans, then we have the beginnings of what can turn out to be this worldwide epidemic problem that the experts call 'pandemics,'" Redlener says.
That is exactly what happened in 1918 when the global epidemic called the Spanish flu struck.
"The Spanish flu was killing people in two or three days once they got sick," said Bill Karesh of the Wildlife Conservation Society.
"In 1918, my now-quite-elderly uncle was a young boy, living in Baltimore, Maryland," says Garrett of the Council on Foreign Relations. "And the flu came through, and his family insisted that he could not go outside for any reason until the whole epidemic was over. He spent afternoons looking out the window and counting the hearses going up and down the neighborhood and trying to guess which of his schoolmates had died."
Unlike the avian flu, the Spanish flu spread long before the international air travel routes of today. At that time, there were no nonstop flights from flu ground zero to the United States. But not anymore.
Karesh believes the avian flu could travel from China to Japan to New York to San Francisco within the first week.
"It's on people's hands. You shake hands. You touch a doorknob that somebody recently touched," Garrett says, referring to how the flu is spread.
Redlener, who is stationed at Mailman School of Public Health at Columbia University, has been working with New York City officials to get ready for the deadly epidemic.
"The city would look like a science fiction movie," according to him. "It's extremely possible we'd have to quarantine hospitals. We'd have to quarantine sections of the city."
"I could imagine that you could look at Grand Central Station and not see much of anybody wandering around at all," Garrett agrees. "People would be afraid to take the subways, because who wants to be in an enclosed air space with a whole lot of strangers, never knowing which ones are carrying the flu?"
As for the hospitals, there would be scenes like the ones this past month in the stadiums of New Orleans and Houston after Hurricane Katrina.
"There wouldn't be equipment and personnel to staff them adequately that you could really call them a hospital," Garrett predicts. "You might more or less call them warehouses for the ailing."
And, as happened in New Orleans, there would be no place for the dead.
"If you look at the expected number of deaths that could occur in cities across the United States, we are wholly unprepared to process those bodies in a dignified and respectful way," asserts Michael Osterholm, director of the Center for Infectious Disease Research and Policy. "We will run out of caskets literally within days."
The prospects have become so bleak that in planning meetings held in New York City, veteran emergency responders have walked away.
"They just don't know how we're going to get through," says Osterholm of those responders. "If we have a repeat of the 1918 life experience, I can't imagine anything to be closer to a living hell than that experience of 12 to 24 months of pandemic influenza."
If the flu does strike, victims at first would not know if it is the kind of easily treated flu that comes every year or the killer flu, known as H5N1.
The man in charge of making sure Americans are prepared in the event of a killer flu epidemic is the secretary of Health and Human Services.
"We would do all we could to quarantine," says Secretary Michael Leavitt. "It's not a happy thought. It's something that keeps the president of the United States awake. It keeps me awake."
The preparedness plan calls for Leavitt to run operations out of a crisis room in Washington.
When pressed as to how ready the country actually is, Leavitt replied, "Not as prepared as we need to be. We're better prepared than we were yesterday; we'll be better prepared tomorrow than we are today."
The draft report of the federal government's emergency plan, obtained and examined by ABC News' "Primetime," predicts as many as 200,000 Americans will die within a few months. This is considered a conservative estimate.
"The first thing is everybody in America's going to say, 'Where's the vaccine?' And they're going to find out that it's really darned hard to make a vaccine. It takes a really long time," said Garrett of the Council on Foreign Relations.
In fact, the draft report says it will not be until six months after the first outbreak that any vaccine will be available, and then only in a limited supply.
"I imagine that not a lot of poor people will get vaccinated," Garrett says. "If you think about New Orleans, this is a similar situation."
While there is no vaccine to stop the flu, there is one medicine to treat it. Called Tamiflu, it is made by the Roche pharmaceutical company in Switzerland. Roche has been selling Tamiflu for years.
Only recently, however, did scientists learn of its potential to work against the killer flu, H5N1. That has since created a huge demand and a critical shortage.
"All of the wealthiest countries in the world are trying to purchase stockpiles of Tamiflu," says Garrett. "Our current stockpile is around 2.5 million courses of treatment."
According to Leavitt, that is a long way from the country's ideal stockpile. "Our objective is to have 20 million doses of Tamiflu or enough for 20 million people," he says.
He later admitted that only 2 million are currently on hand, but asserted that no other country is in a better position.
Officials in Australia, however, have 3.5 million courses of treatment, and in Great Britain, officials say they have ordered enough to cover a quarter of their population.
"I think at the moment, with 2.5 million doses, you are pretty vulnerable," warns professor John Oxford of the Royal London Hospital.
"The lack of advanced planning up until the moment in the United States, in the sense of not having a huge stockpile I think your citizens deserve, has surprised me and has dismayed me," he admits.
Faced with worldwide demand, the Roche company, which produces Tamiflu, has organized a first-come, first-served waiting list. The United States is nowhere near the top.
"The way we are approaching the discussions with governments is that we are operating on a first-come, first-serve basis," says Dr. David Reddy, head of the pandemic task force at Roche.
"Do we wish we had ordered it sooner and more of it? I suspect one could say yes," admits Leavitt. "Are we moving rapidly to assure that we have it? The answer is also yes."
When asked why the United States did not place its orders for Tamiflu sooner, Leavitt replied, "I can't answer that. I don't know the answer to that."
Even leading Republicans in Congress say the Bush administration has not handled the planning for a possible flu epidemic well.
Senate Majority Leader Bill Frist, R-Tenn., says the current Tamiflu stockpile of 2 million could spell disaster.
"That's totally inadequate. Totally inadequate today," says Frist, who is also a physician. "The Tamiflu is what people would go after. It's what you're going to ask for, I'm going to ask for, immediately."
Leavitt says deciding who gets the 2.5 million doses of Tamiflu currently on hand in the United States is part of the federal government's response plan. However, he also admits that thought has motivated the government to move rapidly in securing more doses of the medicine.
"It isn't going to happen tomorrow, but if it happened the day after that, we would not be in as good as a position as we will be in six months," he says.
However, in the end, even the country's top health officials concede that a killer flu epidemic this winter would make the scenes of Katrina pale in comparison.
"You know, I was down in New Orleans in that crowded airport now a couple weeks ago," Frist says. "And this could be not just equal to that, but many multiple times that. Hundreds of people laid out, all dying, because there was no therapy. And a lot of people don't realize for this avian flu virus, there will be very little effective therapy available early on."
ABC News' Rhonda Schwartz, Michael Bicks, Samantha Chapman, Maddy Sauer, Simon Surowicz, Jill Rackmill, Steve Baker, Monica DelaRosa and Jennifer Needleman contributed to this report.