Plague is the greatest killer in human history, though it has only emerged occasionally, most famously in Justinian Rome and medieval Europe. Normally, it moves sluggishly from animal reservoirs into human populations, and it shows little capacity for epidemic spread. Yet under the right circumstances, it is the single most dangerous germ on the planet. Orent reveals how Soviet scientists created genetically-altered forms of this terrible affliction, knowing exactly how to convert plague into a deadly weapon. She shows how scientists are still unable to defend against it, and how plague could be visited upon humanity again.