Disease X

n. A dire contagion requiring immediate attention—but which we don’t yet know about.

In 2013 a virus jumped from an animal to a child in a remote Guinean village. Three years later, more than 11,000 people in six countries were dead. Devastating—and Ebola was a well-studied disease. What may strike next, the World Health Organization fears, is something no doctor has ever heard of, let alone knows how to treat. It’s come to be known as Disease X.

Since René Descartes adopted the letter x to denote a variable in his 1637 treatise on geometry, it has suggested unknowability: the mysterious nature of x-rays, the uncertain values of Generation X, the conspiratorial fantasies of The X-Files. It’s also been used as code for experimental—in the names, for instance, of fighter jets and submarines. That’s an apt association: Disease X may leapfrog from animals to humans like Ebola, but it could instead be engineered in a lab by some rogue state.

Still, far from asking us to resign ourselves to an unpredictable future horror, Disease X is a warning to prepare for the worst possible scenario as best we can. It calls for nimble response teams (a critical failure in the Ebola epidemic) and broad-spectrum solutions. The WHO has solicited ideas for “platform technologies,” like plug-and-play systems that can create new vaccines in months instead of years. As Descartes showed us in mathematics, only by identifying an unknown can we begin to find an answer.