I was in the ballroom of the Mandarin Oriental in Washington, D.C., when the worst pandemic since the 1918 Spanish flu broke out. On cable news, there were reports of four hundred confirmed cases, mostly clustered in Frankfurt, Germany, but with infected individuals reported as far afield as Tokyo, Kabul, and Caracas. Brow furrowed, eyes widened, the anchorwoman’s tone was urgent as she described the spread of a new type of parainfluenza virus, called Clade X. Transmitted through inhalation, it left the infected contagious but otherwise unaffected for up to week before killing more than ten per cent of its victims.

In the ballroom with me, seated around a U-shaped table under glittering chandeliers, were ten senior political figures, an ad-hoc working group convened at the President’s request. The situation looked bad. At Ramstein Air Base, in southwest Germany, three U.S. service members were critically ill, and three infected Venezuelans warned that the outbreak there was much worse than authorities were admitting. According to the Centers for Disease Control and Prevention, a vaccine would likely take more than a year to develop. Meanwhile, Australia, China, and South Africa had already imposed travel restrictions on flights from Germany and Venezuela. A bipartisan group of senators was calling for a similar travel ban; a recent poll had suggested that sixty-five per cent of the public supported them. “What should our priorities be?” the national-security adviser asked.

The room was divided. The health experts at the table warned against closing the borders. “This is an opportunity to do something that will achieve nothing except for diverting resources from real problems,” the Secretary of Homeland Security said. The director of the C.D.C. noted that disrupting global supply chains was a terrible idea, given that most personal protective equipment is imported from overseas. The Secretary of Defense was not convinced. “Look, I’m not talking about going out with our hands waving and our hair on fire,” he said. “But if the President is seen as not taking vigorous action, they’re going to start firing from the Hill.”

Fortunately—and, as you might have guessed by now—this was a fictional outbreak. “Clade X” was a daylong pandemic simulation held by the Johns Hopkins Center for Health Security, in May. The C.D.C. director was being played by a former holder of that job, the Secretary of Defense was the former Republican senator Jim Talent, and the Secretary of Homeland Security was played by Tara O’Toole, a former Under-Secretary in that department. Clade X turned out to be an engineered bioweapon, combining the virulence of Nipah virus with parainfluenza’s ease of transmission. It had been intentionally released by A Brighter Dawn, a fictitious group modelled on the Japanese doomsday cult Aum Shinrikyo, which carried out the sarin-gas attacks in the Tokyo subway system, in 1995. A Brighter Dawn’s stated goal was to reduce the world’s population to pre-industrial levels. By the end of the day, which represented twenty months in the simulation, they had managed to kill a perfectly respectable hundred and fifty million people. “America was just wiped out,” Talent said, before heading to a post-event cocktail reception.

“Clade X” was a daylong pandemic simulation held by the Johns Hopkins Center for Health Security, in May. Photograph Courtesy Larry Canner / Johns Hopkins Center for Health Security

This is the third major pandemic exercise that the Center for Health Security has run. The first, called Dark Winter, was held in 2001 and simulated a smallpox attack on Oklahoma. Its timing, just a few months before 9/11, made its terrifying outcome—the near-complete breakdown of government and civil society—deeply resonant. Dark Winter is credited, in part, with spurring George W. Bush to pass Directive 51, a largely classified plan to insure the continuity of government in the event of a “catastrophic emergency.”

“I would say there has been enormous progress in our preparedness since then,” Tom Inglesby, the center’s director, who played the national-security adviser in Clade X, told me. These advances include new preparedness programs and offices at the C.D.C. and the Department of Health and Human Services; national stockpiling of vaccines and medications; and, at the international level, investments in emergency financing and infectious-disease infrastructure. According to an article that appeared last year in the British Medical Journal, however, the world remains “grossly underprepared.” Philanthropist-in-chief Bill Gates drew on models developed by the Institute for Disease Modeling, a venture founded by his former Microsoft colleague Nathan Myhrvold, to warn that, at our current state of readiness, roughly thirty-three million people would die within the first six months of a global pandemic similar to the 1918 flu.

“Even with the progress that’s been made, there are a lot of serious vulnerabilities,” Inglesby told me. “The reason we wanted to do this now is because there are new leaders, and there are still large issues that have not been resolved.” Some of these issues emerged during the Ebola crisis of 2014, when the United States came very close to shutting down air travel from the West African countries where the disease had broken out. Such a ban would have been in violation of the World Health Organization’s International Health Regulations, to which the U.S. is a signatory; it also, in the opinion of most health professionals, would be both ineffective and counterproductive. “To contain a nasty outbreak like that, you need to move people; you need to move assets; you need labs, equipment, supplies, medicine, food all coming into West Africa,” Inglesby explained.

But, while scientific opinion on the wisdom of travel bans is established, political and popular instinct lags behind—and it was here that the value of Clade X became apparent. As the experts at the table discussed the issue, it was clear that even those who initially leaned toward a ban, such as Talent, were perfectly willing to follow expert advice; they were just extremely sensitive to the pressures of public perception. Telling the average American that a travel ban wouldn’t work “is not going to wash,” Talent said. “This needs to be communicated in terms of their health—that a travel ban will detract from our ability to protect their health.” None of the health professionals at the table had thought to frame their opposition to the ban this way, which left the politicians without the rhetoric they needed to reassure their constituents.

Many of the day’s surprises involved the triumph of these kinds of intangible considerations over rational decision-making. Presented with a scenario in which limited supplies of the vaccine were going to be given to essential workers, including members of Congress and business leaders, rather than children and pregnant women, the table revolted. As acting Attorney General Jamie Gorelick put it, the correct advice may well be to put your own facemask on first before helping others—but leaving babies unprotected is not an option. Meanwhile, faced with a request for military support from Jordan, whose government, weakened by the outbreak, was under siege from terrorist groups, the group was unanimous in agreeing that the United States must continue to support its allies in the Middle East, despite the medical risk and the fact that eighty per cent of Americans thought that the mission was a bad idea. “A number of the players, a number of times, said the equivalent of ‘We can’t do this; it’s not consistent with American values,’ ” Inglesby said. “When you’re having a public-health debate about quarantine, say, or resource allocation, you don’t always go to that level.”