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 says.

Dr. Julie L. Gerberding, head of the 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 it is highly likely that the virus will evolve into a pathogen deadly for humans.

She made the remarks 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, said Gerberding. 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.

“We are expecting more human cases over the next few weeks because this is high season for avian influenza in that part of the world,” said Gerberding. Although cases of human-to-human transmission have been rare, “our assessment is that this is a very high threat.”