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.