Robert G. Webster is one of the few bird flu experts confident enough to answer the key question: Will the avian flu switch from posing a terrible hazard to birds to becoming a real threat to humans?
There are "about even odds at this time for the virus to learn how to transmit human to human," he told ABC's "World News Tonight." Webster, the Rosemary Thomas Chair at St. Jude Children's Research Hospital in Memphis, Tenn., is credited as the first scientist to find the link between human flu and bird flu.
Webster and his team of scientists are working to find a way to beat the virus if it morphs. He has even been dubbed the Flu Hunter.
Right now, H5N1, a type of avian influenza virus, has confined itself to birds. It can be transmitted from bird to human but only by direct contact with the droppings and excretions of infected birds.
But viruses mutate, and the big fear among the world's scientists is that the bird flu virus will join the human flu virus, change its genetic code and emerge as a new and deadly flu that can spread through the air from human to human.
If the virus does mutate, it does not necessarily mean it will be as deadly to people as it is to birds. But experts such as Webster say they must prepare for the worst.
"I personally believe it will happen and make personal preparations," said Webster, who has stored a three-month supply of food and water at his home in case of an outbreak.
"Society just can't accept the idea that 50 percent of the population could die. And I think we have to face that possibility," Webster said. "I'm sorry if I'm making people a little frightened, but I feel it's my role."
Most scientists won't put it that bluntly, but many acknowledge that Webster could be right about the flu becoming transmissible among humans, even though they believe the 50 percent figure could be too high.
Researcher Dr. Anne Moscona at New York Weill Cornell Medical Center said that a human form may not mutate this year or next -- or ever -- but it would be foolish to ignore the dire consequences if it did.
"If bird flu becomes not bird flu but mutates into a form that can be transmitted between humans, we could then have a spread like wildfire across the globe," Moscona said.
No one knows how long or how many mutation changes it would take for bird flu to become a direct threat to humans.
"It may not do it. There may just be too many changes. The virus may not be able to be a human virus," Moscona said.
But that hasn't stopped Moscona from searching for new types of anti-viral treatments that both prevent and slow the spread of bird flu.
"I don't think that once we have human-to-human transmission, it's going to be possible to contain it," she said.
That is why nearly every viral scientist in America, perhaps the world, is waiting and watching the avian flu virus to see if it remains just a threat to birds or changes its genetic code and becomes a deadly threat to humans as well.