WASHINGTON — Quarantine in a 21st-century flu pandemic would look very different from the medieval stereotype of diseased outcasts locked in a do-not-enter zone.

The specter of a military-enforced mass quarantine, raised by President Bush last Tuesday, is prompting debate as health officials update the nation's plan for battling a pandemic — a plan expected to define who decides when and how to separate the contagious from everyone else.

"All the options need to be on the table," said Dr. Marty Cetron, head of quarantine at Atlanta's Centers for Disease Control and Prevention (CDC).

Bush's comments recall how quarantines were enforced in the 1890s, when armed guards patrolled streets to keep victims of smallpox and other dreaded diseases confined to their homes.

"The image that perhaps was inadvertently conveyed is really a setting in extreme that's less likely," Cetron cautioned. "There's a whole range of options in the public-health toolbox for ways to achieve this goal of social distancing."

For three years the CDC has been helping states plan how they would enact quarantines in case of a bioterrorism attack. The instructions stress using the least restrictive means necessary to stem the spread of an infection.

U.S. public-health officials expect a quarantine today to be voluntary, with incentives to cooperate. In case of a horrific outbreak, quarantined areas would get the first shipments of scarce medicines.

The 2003 outbreak of SARS (Severe Acute Respiratory Syndrome) illustrated that "the public will voluntarily comply with measures to both protect themselves and their loved ones" — if doctors make the case that the steps are for their own good, Cetron said.

Vaccination is the cornerstone of fighting a pandemic, and quarantine-like steps are supposed to be brief, "designed to buy time until we have an adequate supply of countermeasures," he said.

Dr. William Schaffner of Vanderbilt University, an influenza expert who advises the federal government, said: "I don't think either the Tennessee National Guard or the U.S. Army and Marines will try to establish a cordon around Nashville. That's not going to happen."

Actually, "we practice in this country quarantine every day," said Health and Human Services Secretary Mike Leavitt. "If a child gets the measles, their mothers are expected to keep them at home."

Legally, "isolation" is the term for separating people who already are sick from others. That happens routinely in hospitals, as access to patients being treated for certain infections is limited.

"Quarantine" means restricting the movement of still healthy people who may have been exposed to an infectious disease, in case they're carrying it. It's almost always for a brief time; during SARS, for instance, hospital workers believed to have been exposed were asked to stay home during the respiratory disease's 10-day incubation period.

States have the primary legal authority to enact quarantines during outbreaks within their borders.

Federal quarantine authority involves preventing infectious diseases from entering the country and stopping interstate spread. Expanding that authority to encompass a military role might entail legislation, something lawmakers' staffs have begun mulling even as public-health experts downplay the need.

With SARS, the CDC used its existing authority to stop that virus from spreading here like it did in Asia: Over three months, CDC workers delayed 12,000 airplanes carrying 3 million passengers arriving from SARS-affected countries. Anyone with SARS symptoms was isolated. Anyone possibly exposed was told what symptoms to watch for in the next 10 days and how to seek help without exposing entire emergency rooms if symptoms arose.

The SARS outbreak showed that tracking down patients and people they may have exposed — allowing individuals, not large areas, to be contained — can work, Cetron said.

At the same time, a super-flu would demand more intense measures because it would spread more easily, perhaps even before symptoms appeared. Drafts of the pandemic plan make clear that affected communities probably would close schools, shut down large gatherings and restrict travel.

Ramping up gradually is crucial to minimize social and economic fallout, Schaffner said.

He offered his home city of Nashville as an example: Authorities first might urge people to watch the Titans play football on TV instead of at the stadium, and to avoid shopping malls. Then schools might close for a while. Then people around the country might be told to postpone holidays or business trips to Nashville, all steps to stem transmission by minimizing contact — but well short of compulsory quarantine.