Even the most careless of Nietzsche's readers – and there have been many – cannot fail to notice the prevalence of biological and medical metaphor in his writings. All too often his predilection for the rhetoric of health and sickness has been portrayed as an idiosyncratic response to, and preoccupation with, his own well-documented medical crises. This is at least partially true: his chronic illness undoubtedly shaped his perception of the world and left an indelible imprint on his thought. But such an approach necessarily ignores the fact that Nietzsche's texts are informed by the same hopes and anxieties that haunted the fin-de-siècle Europe in which he lived, an increasingly medicalised culture that was obsessed with defining and policing the frontiers of the normal and the pathological. His work, which both espouses an anti-Darwinian theory of evolution and evinces an enduring concern with the decadence of Western civilisation, was not immune from the influence of what the neo-Kantian philosopher Heinrich Rickert termed the ‘biologism’ of the late nineteenth and early twentieth centuries – the dissemination of the language of evolutionary naturalism and racial degeneration beyond the boundaries of the rapidly specialising biomedical disciplines and into the wider cultural debates of ethics, politics, anthropology, history and aesthetics. It is my contention that Nietzsche's recourse to biological and medical idiom is both a reflection and an ironic distortion of this pervasive biologism, and can only be truly appreciated once the contemporary force and significance of his metaphor is reconstructed.