A doomsday fungus known as Bd has condemned more species to extinction than any other pathogen. A century ago, a strain of pandemic flu killed up to 100 million people—5 percent of the world’s population. In 2013, a new mystery illness swept the western coast of North America, causing starfish to disintegrate. In 2015, a big-nosed Asian antelope known as the saiga lost two-thirds of its population—some 200,000 individuals—to what now looks to be a bacterial infection. But none of these devastating infections comes close to the destructive power of Bd—a singularly apocalyptic fungus that’s unrivaled in its ability not...