Bird flu risk hinges on computation, mutation.

Byline: Mike Dorning

Nov. 1--WASHINGTON -- News reports on avian flu have taken on alarming tones as the virus has spread from chickens and ducks in Asia to swans in Croatia and, most recently, a parrot at an airport in England. But the risk that the disease poses to people remains frustratingly uncertain.

Even now, no one is sure whether the virus will ever mutate into a disease that can spread easily from person to person and create the potential for a large-scale epidemic. Even if it does, it's not clear whether that would take months, years, a decade or longer.

The threat has seized the attention of official Washington, and President Bush plans to outline what his administration has done to prepare in a speech Tuesday at the National Institutes of Health.

But as much art as science influences the dire estimates that have been quoted for weeks in hundreds of news reports on the potential death toll. Projections vary widely, in large part because of differing assumptions used as the basis for such forecasts. Two estimates in particular have grabbed headlines. The New York Times reported a figure of up to 1.9 million deaths in the U.S. from a draft government report. And a highly publicized estimate of 541,000 deaths came from a public health advocacy group based in Washington.

But an earlier, less calamitous estimate by a scientist at the Centers for Disease Control projected deaths in a range between 89,000 and 207,000…