A bird flu virus may mutate to a human form that becomes as deadly as the ones that killed millions during three influenza pandemics of the 20th century.
"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," Gerberding said in remarks at the national meeting of the American Association for the Advancement of Science.
Although cases of human-to-human transmission have been rare, "our assessment is that this is a very high threat" based on the known history of the flu virus, she said.
In Asia, there have already been a number of deaths among people who caught the flu from chickens or ducks.
The avian flu now spreading in Asia is part of what is called the H1 family of flu viruses.
There had been small appearances of the H1-type of avian viruses in other years, but nothing like the H5 now rampaging through the birds of Asia.
"We are seeing a highly pathogenic strain of influenza virus emerge to an extraordinary proportion across the entire western component of Asia," she said.
Study already has shown that the virus can infect cats who can then infect other cats, which Gerberding said was "another harbinger" of the possibility of a human pandemic.
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.
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.