Federal officials sent states detailed guidelines yesterday for rapidly vaccinating millions of people should terrorists attack using deadly smallpox.

Because no outbreak of smallpox has occurred for decades, health officials assume that even a single case of the disease would mean the nation is under attack.

They would immediately vaccinate those who had contact with the highly infectious patient in hopes of stemming the spread of the virus.

Experts then would assess the scope of the attack to determine how many others need protection, officials at the federal Centers for Disease Control and Prevention said yesterday.

"Once there is a case of smallpox, what has been theoretical up 'til now suddenly becomes real. We would anticipate many Americans would want to have access to the vaccine," said Dr. Julie L. Gerberding, director of the CDC.

The plan posted yesterday on CDC's Web site offers specific suggestions for a community that must vaccinate 1 million people in 10 days, but officials said the numbers and timing would depend on the situation.

Last month, Dr. D.A. Henderson, a top bioterrorism adviser to the federal government, said many communities are woefully unprepared for the task. Gerberding agreed. In most jurisdictions, there's a long way to go, she said.

The 48 pages of guidelines are meant to help states confront a host of logistical problems.

Although smallpox was declared eradicated in 1980, experts fear that hostile nations or terrorist groups might have acquired the virus. Because routine immunization in the United States ended in 1971, the population is considered very vulnerable to a smallpox attack.

The new blueprint does not address a thornier issue now under intense discussion within the Bush administration: whom to vaccinate before an attack occurs. They tentatively have settled on a staged approach, beginning with health care workers and emergency personnel who would have to respond to an outbreak.

No decision has been made on whether to make the vaccine available to the public on a voluntary basis.

Officials are trying to balance the risk of serious side effects from the vaccine, including death, against the risk of the disease's return.

The blueprint released yesterday includes consent forms listing vaccine side effects, public service announcements for the media, details on storage and preparation of the vaccine and staffing guidelines to open clinics 16 hours a day.

The material lists categories of people, including those infected with HIV or who suffer from eczema, for whom the vaccine could be dangerous.

Maryland Health Secretary Dr. Georges Benjamin said the blueprint will be useful as state officials make plans to cope with biological attacks.

"We're building the capacity to do large-scale vaccination," he said. "If it happened tomorrow, we could do it, but it would be tough."

The smallpox vaccine offers protection against the disease even if administered after someone is exposed, as long as that person gets the shot within a few days.