On the outskirts of Madison, Wisconsin, stands a low brick structure equipped with ventilation scrubbers and surrounded by a tall chain-link fence: the Tight Isolation Building of the U.S. Geological Survey's National Wildlife Health Center (NWHC), a federal research facility devoted to combating wildlife diseases. Inside, a cinder block corridor circuits the Animal Isolation Wing, passing a series of well-sealed experiment rooms, each visible through a thick window. One room is furnished with sawdust and burrowlike pipes to approximate the habitat for prairie dogs involved in a vaccine trial against Yersinia pestis, the organism that causes plague. In another room zebra finches in birdcages are playing a role in research toward a vaccine for West Nile virus. Two rooms are darkened, for the comfort of hibernating bats. The first contains normal animals of the species Myotis lucifugus, commonly called little brown bats. They are the controls. The second dark room houses little browns exposed to Geomyces destructans, a filamentous white fungus of unknown origin that first appeared among North American bats in 2006. In just four years, it has hit hibernating bat populations in New York, Vermont, and a growing list of other states and Canadian provinces more lethally than Yersinia pestis hit the peasants of medieval France.