Ready or not, here it comes:  It is being spread much faster than first predicted from one wild flock of birds to another, an airborne delivery system that no government can stop.

"There's no way you can protect the United States by building a big cage around it and preventing wild birds from flying in and out," U.S. Secretary of Agriculture Michael Johanns said.

U.S. spy satellites are tracking the infected flocks, which started in Asia and are now heading north to Siberia and Alaska where they will soon mingle with flocks from the North American flyways.

"What we're watching in real time is evolution," said Laurie Garrett, a senior fellow for global health at the Council on Foreign Relations. "And it's a biological process, and it is, by definition, unpredictable."

Industry Precautions

America's poultry farms could become ground zero as infected flocks fly over. The industry says it is prepared for quick action.

"All the birds involved in it would be destroyed, and the area would be isolated and quarantined," said Richard Lobb of the National Chicken Council. "It would very much [look] like a sort of military operation if it came to that."

Extraordinary precautions are already being taken at the huge chicken farms in Lancaster County, Pa., the site of the last great outbreak of a similar bird flu 20 years ago.

Other than the farmers, everyone there has to dress as if it were a visit to a hospital operating room.

"Back in 1983-1984, we had to kill 17 million birds at a cost of $60 million," said Dr. Sherrill Davison, a veterinary medicine expert at the University of Pennsylvania.

Can It Be Stopped?

Even on a model farm, ABC News saw a pond just outside the protected barns attracting wild geese

It is the droppings of infected waterfowl that carry the virus.

The bird flu virus, to date, is still not easily transmitted to humans. There have been lots of dead birds on three continents, but so far fewer than 100 reported human deaths. [103 as of Mar. 21]

But should that change, the spread could be rapid.

ABC News has obtained a mathematical projection prepared by federal scientists based on an initial outbreak on an East Coast chicken farm in which humans are infected. Within three months, with no vaccine, almost half of the country would have the flu.

That, of course, is a worst-case scenario — one that Lobb says the poultry industry is determined to prevent with an aggressive strategy to contain and destroy infected flocks and deny the virus the opportunity to mutate to a more dangerous form but one that experts say cannot be completely discounted.

The current bird flu strain has been around for at least 10 years and has taken surprising twists and turns — not the least of which is that it's now showing up in cats in Europe, where officials are advising owners to bring their cats inside. It's advice that might soon have to be considered here.

Press "Replay" to start over.