Federal officials sent states detailed guidelines Monday for rapidly vaccinating their entire populations against smallpox should the deadly disease return through an act of terrorism.

It's been decades since smallpox was seen in this country and the disease has been eradicated from Earth, so officials would assume that a single case of smallpox means 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 disease.

Experts then would assess the scope of the attack -- including the number of patients, circumstances of exposure and details of their disease -- to determine how many others need protection, officials said Monday. It's possible that even a single patient would trigger vaccinations across the entire country, they said.

"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," Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, said in an interview.

At the same time, a single case suggests there will probably be other cases, increasing the risk and suggesting the need for "a fairly widespread control program," said Dr. Walter Orenstein, director of the CDC's National Immunization Program.

The plan sent Monday offers specific suggestions for a community that must vaccinate 1 million people in 10 days, but officials said Monday that the timing and the scope of vaccinations will depend on the situation.

"The decision around mass vaccination would be dependent on particulars of the outbreak we were facing," said Dr. David Fleming, the CDC's deputy director for science. But he added, "We've got to develop the plans right now."

Just 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 ways to go," she said Monday.

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 may have acquired the virus and could release it on a population that now harbors very little natural immunity. Routine immunizations in the United States ended in 1971.

The disease centuries ago swept through entire population, killing up to 30 percent of its victims and leaving survivors scarred and sometimes blinded. The vaccine was discovered in 1796.

The new blueprint does not address a thornier issue now under intense discussion within the Bush administration: whom to vaccinate before an attack even occurs. They tentatively have settled on a staged approach, beginning with health care workers and emergency responders who face the greatest risk of seeing a contagious smallpox patient. A decision on that issue is expected by month's end.

Because the vaccine carries significant risks, including death, officials are trying to balance the risk of the side effects against the risk of the disease's return.

The blueprint released Monday gives states considerable information about how to deliver shots to large number of people all at once. It includes:

-- Ready-to-go medical screening, consent forms and information for distribution about smallpox and the side effects of the vaccine.

-- Details about how to store and prepare the vaccine for use.

-- A list of places where security or crowd control will be needed, including clinics and vaccine storage sites.

-- Types and numbers of workers needed per clinic, per 8- and 16-hour shifts. Among the required workers: volunteers to run an informational video, nurses to give the shots and translators to assist non-English speakers.

-- Messages to deliver to people waiting for shots, including how long the wait is. And messages for the media, including immediate public service announcements.

-- Criteria for choosing a vaccine clinic site before a smallpox attack.

-- Issues to consider in transporting people to the site, including buses and parking.

-- A long list of needed routine and emergency supplies, from tape and rubber bands to cots and blankets to fax machines and VCRs.

-- Information about who should not receive the vaccine due to heightened risk of side effects.

The mass vaccination planning is meant as a supplement to the "ring vaccination" plans that have been circulating since last year. If a smallpox patient were discovered, authorities would first deliver shots to those closest to the contagious patient. They would then work away from the central smallpox patient, vaccinating people the patient may have exposed and then others whom those people may have exposed.

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.