This paper explores the semantic instability of the term “contagion” in the nineteenth century as refracted through theatre and performance, with key examples as case studies. I’ll look at 19th-century theatrical engagements with evolution, biology, and other related sciences, to show theatre’s preoccupation with mechanisms of transmission broadly conceived—from imaginative versions of heredity (including telegony in Ibsen and Strindberg, for instance) to breastfeeding on stage in Herne and Brieux to the “contagious” theatricality at the heart of Charlotte Mew’s short story “A White Night.” These and other examples can help us think about how and when the line began to blur between a strictly medical definition of contagion and a fuzzier “social disease” usage, onto which theatre cottoned very early on. I will then trace the powerful legacy of these theatrical engagements with contagion, looking first at how Artaud radically extends earlier metaphoric uses of contagion into his immersive, experiential “plague” and finally exploring the present day in which virtual contagion games allow the user to “perform” plagues and pandemics. A unifying thread running through all of these examples is how contagion relates to definitions of culture (e.g. Greenblatt, Foucault) founded, paradoxically, on containment and control. Finally, I will explore briefly how all of this relates to the wider issue of how to forge productive disciplinary cross-contaminations in a professional environment that increasingly regulates, directs, and manages trans- or interdisciplinarity.