The medical-humanities literature on zoonosis has overwhelmingly stressed the manner in which cross-species diseases challenge anthropocentric accounts of society. This article explores the colonial discovery, between 1896 and 1910, that bubonic plague (the disease responsible for the medieval Black Death) was zoonotic. This scientific work involved a massive, almost industrialised, examination of rat corpses so as to produce an animal linkage in plague. I show that this production of animal agency paradoxically served to hasten an ongoing process whereby human behaviour was identified as the moral locus of disease. The scientific production of zoonosis thus enabled another form of reasoning and judging to come to the fore, which ultimately had the effect of strengthening and heightening existing racial stereotypes and hierarchies. Far from challenging an anthropocentric worldview, this zoonosis helped to re-establish one.