The group’s task: respond to a fictional outbreak. A virus is killing dozens in Frankfurt, Germany, and spreading in Venezuela, though that country’s president denies the problem. It’s moving fast and has a high mortality rate. The leaders immediately must decide whether to shut airports (they don’t) and give assistance to Venezuela (they do), and how to calm the public as fake news spreads paranoia on social media.

First, though, they have to understand the enemy. In the real world, says Jonathan Quick, a doctor who attended the exercise and is the author of a book on preparedness, The End of Epidemics, three out of five novel diseases come from “the bush or the barn.” That is, like Ebola and SARS, they make the jump to humans from animals. In last week’s scenario, the players also initially suspect a zoonotic source but quickly learn that this disease doesn’t fit any known family of viruses, called clades. Could it be man-made?

Eric Toner, a physician specializing in pandemics, designed a “germ game” in which terrorists release a man-made virus. Larry Canner/Johns Hopkins Center for Health Security

Indeed it is. Someone has genetically modified a mostly harmless parainfluenza virus to kill. The fictional culprit is A Brighter Dawn, a shadowy group promoting the philosophy that fewer people—a lot fewer—would be a good thing for planet Earth. In fact, they want the population to return to preindustrial levels.

The scenario was created by Eric Toner, an emergency physician and pandemic specialist with Johns Hopkins University's Center for Health Security, which sponsored the exercise. Toner carried out meticulous research to come up with a plausible threat using real virology and epidemiological models. The result was so realistic that the organizers chose not to present too many details. “For obvious reasons,” he says. “It does not require a nation-state to do it.”

That may be the biggest change since 2001. Since then, genetic engineering has become easier, and powerful tools like CRISPR are easily obtained. “The most fascinating thing is that technology used to be up here,” says Scott Lillibridge, the onetime head of the CDC’s bioterrorism program and now a professor at Texas A&M. “I can tell you in the 1990s we were thinking about state actors. It was a virus in the freezer. Fast-forward 20 years and the appearance of synthetic biology means that things which used to require a major investment are cheap and easy to acquire.”

In the past, it was enough to stockpile vaccines against familiar germs—smallpox, polio, anthrax. But now an evildoer could create new threats not on anyone’s list of bogeymen. As Bill Gates put it this year, “The next epidemic could originate on the computer screen of a terrorist.”