In search of lost fleas: reconsidering Paul-Louis Simond’s contribution to the study of the propagation of plague

Abstract Paul-Louis Simond’s 1898 experiment demonstrating fleas as the vector of plague is today recognised as one of the breakthrough moments in modern epidemiology, as it established the insect-borne transmission of plague. Providing the first exhaustive examination of primary sources from the Institut Pasteur’s 1897–98 ‘India Mission’, including Simond’s notebooks, experiment carnets and correspondence, and cross-examining this material with colonial medical sources from the first years of the third plague pandemic in British India, the article demonstrates that Simond’s engagement with the question of the propagation of plague was much more complex and ambiguous than the teleological story reproduced in established historical works suggests. On the one hand, the article reveals that the famous 1898 experiment was botched, and that Simond’s misreported its ambiguous findings for the Annales de l’Institut Pasteur. On the other hand, the article shows that, in the course of his ‘India Mission’, Simond framed rats as involved in the propagation of plague irreducibly in their relation to other potential sources of infection and not simply in terms of a parasitological mechanism. The article illuminates Simond’s complex epidemiological reasoning about plague transmission, situating it within its proper colonial and epistemological context, and argues for a new historical gaze on the rat as an ‘epidemiological dividual’, which highlights the relational and contingent nature of epidemiological framings of the animal during the third plague pandemic.