LAURIE GARRETT: Each year different flues come, but your immune says, I've seen that guy before, no problem, crank out some antibodies. Might not feel great for a couple of days, but I'll recover. 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.
BRIAN ROSS: This form of the flu started as do almost all flu viruses, in wild birds in Asia.
BILL KARESH, WILDLIFE CONSERVATION SOCIETY: They die of pneumonia just like people. When you open them up, you do a post-mortem exam, their lungs are just full of fluid and full of blood.
BRIAN ROSS: Bill Karesh, the lead veterinarian for the Wildlife Conservation Society, has been tracking this rare flu strain since it first emerged in the 1990s, and has watched it as it's gained strength going from wild birds to chickens to humans.
BILL KARESH: The last outbreak in July that was reported was this part of China. It started in a market somewhere in the Guangdong Province. It's just packed with cages. you'll have chickens, you'll have ducks, cats, dogs, turtles, snakes. And they're all stacked in cages and they're all spreading their germs to each other.
BRIAN ROSS: Asian governments have killed millions of chickens in a futile attempt to stop the flu's spread. Dr. Irwin Redlener heads Columbia University's Center for Disaster Preparedness.
DR. IRWIN REDLENER: The tipping point, the place where it becomes something of immediate concern is when that virus mutates to something that is able to go from human to human.
BRIAN ROSS: So now, scientists in Asia and around the world, on a literally hour-to-hour basis, are watching for that tipping point to occur.
DR. MALIK PEIRIS, RESEARCH SCIENTIST: This virus is a pretty nasty piece of work.
BRIAN ROSS: Dr. Malik Peiris is the scientist who first identified the so- called SARS virus two years ago, which killed more than 700 people and triggered a worldwide scare. This, he says, is much worse.
DR. MALIK PEIRIS: 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, causes a severe pneumonia.
BRIAN ROSS: To date, there have been 57 confirmed human deaths and another suspected one just last week in Indonesia. Scientists say the humans who have been infected so far have been infected by birds, but that every infected person represents one step closer to the tipping point.
DR. IRWIN REDLENER: Once that virus is capable of not needing the bird to infect humans, so in other words, you have it, I can get it from being in contact with you, then we have the beginnings of what can turn out to be this worldwide epidemic problem that the experts call pandemics.
BRIAN ROSS: That's what happened in 1918. A shocking reminder of the worst-case scenario. It was known as the Spanish flu.
DOCTOR: It was the most horrible time. By the hundreds they were dying, and no doctor could stop the epidemic.
DOCTOR: We had little caskets for the little babies that stretched for four and five blocks.
BILL KARESH: The Spanish flu was killing people in two or three days once they got sick. It started at the end of World War I. It actually came to America with returning soldiers from Europe and then spread through the United States.