This chapter addresses a topic – health – that has come to light in the context of the Covid-19 pandemic. Bringing together animal studies and the medical humanities, the chapter examines key texts from Leo Africanus, William Shakespeare, Daniel Defoe, and others to trace close historical and discursive relationships between human and animal health. Focusing largely on diseases thought to be what we would now call zoonotic, or transferable from nonhuman animals to humans, the chapter seeks to make apparent the significance of cross-species infections that, before antibiotics and most vaccines, helped shape literature, trade, health, and imperial order. Although rabies provided the most recognizable model of cross-species infection, the chapter begins with locust swarms, which offered many Christian writers a model for articulating global connections between pests and pestilence, then turns to shipboard rats, which, even before germ theory, were thought by many early moderns to function as harbingers of death and disease. Having demonstrated how deeply cross-species contagion was entangled with theological definitions about what it is be human, the chapter ends by exploring a little known topic – early modern and eighteenth-century cattle plague – and its implications for reimagining a multispecies medical posthumanities.