WASHINGTON, D.C. -- In the space of five hours Friday, the world's leaders learned just how vulnerable modern society is to an old scourge called smallpox.

This was only a simulation, an exercise called "Atlantic Storm" that was designed by academics and conducted in the comfort of a hotel ballroom. But for the participants -- 12 political veterans led by former U.S. Secretary of State Madeleine Albright -- it nevertheless proved chilling.

The hypothesized bioterror attack not only resulted in a rapidly rising number of casualties, it also raised troubling questions about controlling borders, sharing scarce vaccine and preventing a global economic collapse. And it demonstrated that such an attack could quickly rend the fabric of today's global society.

"This was quite a surprising and breathtaking exercise," said Werner Hoyer, a member of the German Bundestag who played the role of the German chancellor during the simulation. "This is something that only a very small number of politicians are aware of.''

In a real-time scenario devised by the Center for Biosecurity of the University of Pittsburgh Medical Center, Hoyer, Albright and other players, including Jerzy Buzek, the former prime minister of Poland, played the roles of the leaders of the major transatlantic powers. By happenstance, they had assembled in Washington for a summit meeting the day after German officials confirmed smallpox in three Turkish visitors.

When they assembled around a U-shaped table at 9 a.m., a few dozen cases had been identified in Germany, Turkey, the Netherlands and Sweden, and an al-Qaeda splinter group had claimed credit for launching the bioterror attack.

But the casualties rose by the hour--240 cases in six countries, then 956 cases and, by 2 p.m., 3,320 cases, including 716 in the United States. Projections suggested 660,000 people would be infected worldwide by mid-February.

"If there was one lesson from today," said Albright, who played the role of the U.S. president, "it's that we should vaccinate everyone--today."

Though a vaccine is available for smallpox, people have not been routinely vaccinated since 1980, when smallpox was considered eradicated. Many nations have virtually no vaccine in stock. No treatment exists for smallpox, which kills about 30 percent of the people it infects.

The Atlantic Storm exercise was developed jointly by the UPMC Center for Biosecurity, a leading bioterrorism policy think tank based in Baltimore, and the Center for Transatlantic Relations at Johns Hopkins University.

Four years ago, the biosecurity center, then affiliated with Johns Hopkins, conducted a two-day exercise called Dark Winter that hypothesized a smallpox attack in Oklahoma. The participants included former Sen. Sam Nunn and then-Oklahoma Gov. Frank Keating. As that scenario played out, smallpox spread to 25 states and 10 foreign countries in 12 days, smallpox vaccine stocks were exhausted, and civil disorder broke out across the nation.

Dark Winter highlighted the lack of U.S. preparedness for bioterrorism. The exercise proved prescient a few months later when anthrax spores were mailed to political and media figures in the weeks following the 9/11 attacks

Atlantic Storm shifted the focus to the international stage.

One of the first issues to face the leaders was how to allocate smallpox vaccine. Some nations, such as the United States, Great Britain and the Netherlands, have stockpiled enough to immunize all of their citizens. But most nations have relatively little on hand.

In the exercise, Turkey made an early demand for NATO to send enough vaccine to immunize its 70 million citizens.

Nations such as Israel and Great Britain, noting the importance of helping a Muslim country, pledged millions of doses, but leaders resisted the demand for mass immunization. Instead, they hewed to the World Health Organization's recommendation to perform "ring immunization"--vaccinating contacts of those infected, as well as health care workers and perhaps transportation workers.

It was an admirable stand, but Dr. Tara O'Toole, director of the biosecurity center, said later that it ultimately would make little difference.

"Time matters," she explained, noting that the long incubation period for smallpox (12 to 14 days) means that once cases begin to appear, the virus has already been in circulation for weeks. So when an attack is recognized, only a limited time remains to distribute vaccine.

The leaders nevertheless spent much of their time discussing the need for the "haves" to share their vaccines with the "have-nots" and how such a strategy ran counter to popular pressures to conduct mass vaccinations. When the first U.S. cases of smallpox appeared during the exercise, for instance, Albright noted that attempts to reallocate U.S. vaccine doses had become problematic.

"It's clear there will be public pressure now for the U.S. to take care of itself," she said.

Concerns also arose about keeping borders open and maintaining the flow of goods and services. Leaders feared that some countries would be tempted to close their borders to travelers from countries with smallpox infections.

Nigel Broomfield, a veteran British diplomat playing the British prime minister, observed that despite the terrorists' intentions to disrupt Europe and North America, smallpox would eventually reach the Muslim world, which is poorly prepared to deal with such a crisis.