The germ theory of disease famously brought a new notion of specificity into concepts of disease. At the same time, the work of Pasteur, Koch and their colleagues was developed during the same decades as Charles Darwin's theories of evolutionary biology challenged traditional notions of the essentialism of biological species. This essay examines some of the ways in which Darwin's work was invoked by British doctors seeking to explain clinical or epidemiological anomalies, in which infectious diseases did not appear to (...) breed true. (shrink)