This essay tells the story of the “Danysz virus,” a bacterial culture that was designed and deployed to cause epidemics among rodents and was sold globally by the Institut Pasteur from 1900. Jean Danysz (1860–1928) initially identified the culture during his studies of epidemics in population cycles of common voles. His experiments turned to the ambitious goal of increasing the bacteria’s virulence by emulating the rodent’s animal economy in the laboratory in order to mass-produce a culture. The bacterial culture was supposed to bring about a man-made epidemic for sale on the global pest- and plague-control market. The essay considers the Danysz virus as a “cognitive good” and analyzes the material as well as intellectual transfers that shaped its development, supported its international application, and prompted the experimental testing of its promises. While the culture largely failed to bring about the exponential growth of lethal infections in rat populations Danysz promised, the virus did succeed in distributing his theoretical revision of virulence. Through its commercial distribution, his product recast virulence as a function of the relation between bacteria and their milieu and offered a novel concept of mutual pathogenicity that far exceeded deterministic models of infection prevalent at the time.