‘Plague’ discusses the identity of plague. The word ‘plague’ has been used over the centuries to denote an epidemic disease of particular severity and dramatic impact, and a disease which probably always had the same causative agent, now known to have been a bacillus, Yersinia pestis. Today, plague is certainly much better understood than it has ever been. The findings of modern medical science about plague’s causes (aetiology) and its incidence and distribution (epidemiology) have illuminated many aspects of its history. However, they have given rise to considerable debate about whether all the plagues of the past can have had the same cause as those of the present.