Serving as an introduction to The Fifth Plague: Cattle, Contagion, and the Medical Posthumanities, this chapter argues for a strategic alliance between the One Health movement, which attends to cross-species epidemics, and the health humanities, which tend to focus on humans alone. I call this approach the medical posthumanities. The Fifth Plague contributes to the medical posthumanities in a specific way: it offers a transdisciplinary history of cattle plagues and their real or imagined cross-species contagions, thereby illuminating the shared vulnerability of humans and livestock in global cattle-grain regimes. That causal explanations for repeated and devastating cattle plagues are rarely unyoked from both classical and Christian disease ecologies make this kind of cross-disciplinary reading necessary for understanding the past, and helpful for appreciating the present; zoonotic diseases circulate around and through us at an alarming rate, their causes and cures still tethered in many ways to Christian scourge logic.