WASHINGTON -- The Earth may be on the brink of a worldwide epidemic from a bird flu virus that may mutate to become as deadly and infectious as viruses that killed millions during three influenza pandemics of the 20th century, a federal health official said yesterday.

Dr. Julie L. Gerberding, head of the U.S. Centers for Disease Control and Prevention, said scientists expect that a flu virus that has swept through chickens and other poultry in Asia will genetically change into a flu that can be transmitted from person to person. The genes of the avian flu change rapidly, she said, and experts believe that it is highly likely that the virus will evolve into a pathogen deadly for humans.

She spoke in a plenary lecture at the national meeting of the American Association for the Advancement of Science.

In Asia, there have already been a number of deaths among people who caught the flu from chickens or ducks. The mortality rate is very high -- about 72 percent of identified patients, Gerberding said. There also have been documented cases of this strain of flu being transferred from person-to-person, but the outbreak was not sustained, she said.

Although cases of human-to-human transmission have been rare, "our assessment is that this is a very high threat," she said.

The avian flu now spreading in Asia is part of what is called the H1 family of flu viruses. It is a pathogen that is notorious in human history.

"We are seeing a highly pathogenic strain of influenza virus emerge to an extraordinary proportion across the entire western component of Asia," she said. "The reason this is so ominous is because of the evolution of flu. You may see the emergence of a new strain to which the human population has no immunity."

The CDC chief said her agency is getting ready for a possible pandemic next year. A special flu team, organized last year, continues to monitor the spread of the avian flu and to analyze the strains as they appear.

The government has ordered 2 million doses of vaccine that would protect against the known strains of avian flu. Gerberding said this would give manufacturers a head start on making the shots that would be needed to combat a full-blown epidemic of an H1-type of flu in this country.