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.