The planning, still in draft form, addresses complex logistical and policy questions, including where people would be kept while waiting for officials to confirm a smallpox case and, if necessary, administer vaccinations.

"It's not pretty to think through these type of doomsday scenarios, but it's important to start to put yourself there and imagine things unfolding if you want to anticipate how to react," said Marty Cetron, a quarantine expert at the Centers for Disease Control and Prevention.

Within the next few weeks, the plan will be circulated among top federal officials and others involved in the preparation, said Cetron, who co-chairs a CDC working group on the issue.

The group began with this scenario: Officials get word that an international flight is headed for the United States with a smallpox-infected passenger aboard. The case would be fairly easy because the people would be in one place and health officials wouldn't have to track them down.

Still, if the passengers weren't held for treatment, CDC experts estimate that a plane filled with 500 people including one with smallpox could result in dozens - maybe hundreds - of sick people, including passengers on the plane who got sick and the people they would infect.

The vaccine is only effective within four days of someone being exposed to smallpox, so if people leave the scene it would be difficult to find them in time.

Under the plan, officials would stress that people would be better off staying in quarantine because that's where the vaccine and other medications would be available. That's a big change from past centuries, when people in quarantine were written off, Cetron said.

"All the goodies will be delivered inside the box," he said. "It's a fundamental difference to spend as much, if not more attention, on monitoring and preserving the health on the people inside the box."

Once planning is complete for this scenario, the CDC group plans to tackle more complicated situations, such as a smallpox patient in a sports stadium or an infected person wandering through an airport.

Smallpox was declared eradicated in 1980, but samples of the deadly virus still exist in both the United States and Russia. Bioterrorism experts consider the possibility of a smallpox attack one of the most frightening, albeit unlikely, threats because one infected patient could infect many others.

Routine immunizations against smallpox in the United States ended in 1972, meaning many Americans have no protection against the disease. Federal officials are now considering vaccinating as many as 500,000 health care workers and emergency personnel who would be first to see any smallpox case. But because the vaccine carries significant risks, including death, officials do not want to resume mass vaccinations.

Rather, the plan is to vaccinate others only if they are exposed to the virus, or exposed to someone who is exposed.

The airliner scenario is not entirely fictitious. In December, a flight from Taiwan bound for Newark, N.J., was delayed for about four hours in Seattle after an anonymous tipster told U.S. Customs Service agents that a passenger had smallpox. It turned out to be a hoax, but the situation provided a real-life example of how people on the plane reacted.

"People got really scared because they sat there on a plane for two hours without any idea what was going on," said Kristy Lillibridge, an epidemiologist who is part of the CDC working group.

The draft CDC plan calls for taking the airplane to a hangar or another facility where passengers can get off without going into the airport, she said. The suspicious passenger would be isolated from the others, but everyone would be kept in quarantine while officials did tests to determine if the passenger was in fact infected with smallpox.

Still unresolved: how long to keep people if a smallpox exposure is confirmed. Options include one to two days, or long enough to give the shots; seven days, long enough to see if the vaccination worked; or 17 days, when no one would be contagious anymore.

"It is such a sensitive topic. Those kinds of decisions, to actually use a quarantine, will be made at very high levels," Cetron said.

Other elements of the plan, officials said, include:

-If possible, diverting the flight to one of eight international airports where the CDC has quarantine operations in place, or to another airport where the local public health authorities are prepared to handle the case.

-Creating clear messages and announcements to deliver to passengers, including those that would be read by the pilot while they are still on the plane.

-Providing beds, restrooms, food and access to medication for anyone in quarantine.

AP-ES-07-08-02 1622EDT