As soon as Bush announces how many Americans should be vaccinated, the CDC will have to explain the policy, educate the public and supervise a complex vaccination campaign that could produce hundreds of serious side effects and even kill some people.

The campaign could prove to be a major trial of the Atlanta-based agency's ability to communicate clearly and rapidly on a public health issue capable of provoking broad national anxiety -- a task that some lawmakers and health experts said the CDC failed to perform well during the anthrax crisis.

Bush's decision arrives at a vulnerable moment for the agency, with the political power of the Democratic Party, often a CDC ally, sharply reduced by the midterm elections, and with director Julie Gerberding still relatively new to her job.

"This is a difficult time for the CDC," said Dr. William L. Roper, who led the agency from 1990 to 1993 and is now dean of the University of North Carolina's school of public health. "The job that [it] has in a post-9/11, post-anthrax world is a monumental task."

The last known victim of the anthrax-tainted letters, Ottilie Lundgren of Connecticut, died one year ago Thursday. The 12 months since have been a chaotic time for the CDC. The agency emerged from the crisis with new responsibilities: joining the top levels of national security planning and making preparations for bioterrorism its top priority.

Since then, the CDC has won points among its close observers -- faculty at major schools of public health, staff at state health departments and members of public health organizations -- for the agency's thorough and straightforward handling of the West Nile virus epidemic, which has caused 3,698 illnesses and 212 deaths this year.

The agency also has an advantage it did not have a year ago: Gerberding is strongly supported by Health and Human Services Secretary Tommy Thompson. Friction between the HHS leadership and the CDC led to the departure in March of director Jeffrey Koplan.

"People know, when she talks, that she has the full backing of the folks in the Humphrey Building," Roper said, referring to Health and Human Services' Washington headquarters.

Gerberding has won praise for moving quickly to cement relationships with Congress and the White House and for speeding up the CDC's painstaking process for approving information that it releases to the public.

"Six months is a short time -- but so far, so good," said Dr. Louis W. Sullivan, who was Health and Human Services secretary under President George H.W. Bush and is president emeritus of Morehouse School of Medicine in Atlanta.

Before the anthrax attacks, CDC staff members often joked that the agency was better known in the developing world than in its own Druid Hills back yard. At the same time, CDC supporters complained that the agency's outside-the-Beltway location kept it from being taken seriously by lawmakers.

Gerberding has moved to change that. In her first months in office, she spent at least one day each week in Washington.

"If we want to remain irrelevant, we can remain invisible and quiet," she said in an interview. "But I am part of the Bush administration. CDC is part of the executive branch. We have a role to play. Rather than hiding from that role, I think we should embrace it and try to shape it."

Last year's anthrax attacks and the fears raised by potential war with Iraq -- with its suspected development of biological weapons -- have raised the CDC's profile within the government and with the public at large.

"The CDC now has a whole other set of constituents, including the national security establishment, the law enforcement agencies at every level of government, the White House staff and the president himself," Roper said. "The focus on the CDC has suddenly gotten much bigger. And its budget is bigger. [It] will come in for greater scrutiny and be held to a higher level of accountability."

The higher profile carries additional political risks.

Come January, Republicans will control both houses of Congress and the presidency. In the past, some social conservatives have taken aim at the CDC's programs focusing on gun violence, AIDS prevention and research into some diseases. They argue such efforts edged outside the agency's historic responsibility for curbing infectious diseases.

However, some Republican lawmakers, including Georgia's Sen.-elect Saxby Chambliss, have been strong supporters of the CDC.

"I think they will do better this time under Republicans than they would have if national security were not such an issue," said R. Gregory Evans, director of the Center for the Study of Bioterrorism and Emerging Infections at St. Louis University.

While its increased responsibility for bioterrorism preparedness may ensure the CDC a friendlier reception on Capitol Hill, some agency watchers warn there is still an appetite in Washington for major change at the CDC.

Some critics say privately that the new director should simplify the agency's complicated structure and create a new management team. Gerberding has hired one new deputy director and a director of a center that is but one of the agency's 12 subdivisions. The rest of the team she inherited remains in place.

Others say it will be impossible for the new director to shape the agency to her liking until she can spend more time there. Current events make that difficult. Last week, for instance, Gerberding missed several days in Atlanta, including a scheduled TV appearance on "Good Morning America" during its broadcast here, when her bosses asked her to extend a Washington visit for several days.

Gerberding suggested in an interview that change is coming to the CDC. She has been taking time, she said, to learn the agency thoroughly since assuming the top job in July.

Unlike many CDC employees, she is not a lifer. She arrived in 1998 and, until the anthrax crisis forced her into prominence, spent most of her time at the agency in a relatively small division.

"I made a disciplined and conscious effort to not come in here and start making changes," she said. "I have been finding it challenging to not take immediate action to address some of the things I think I want to do -- but I am glad I didn't, because I think I have learned a great deal."