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.

Poliomyelitis is a disease whose incidence steadily increased during the second half of the twentieth century on both sides of the Atlantic. If in the United States the epidemics which afflicted young children each summer became a major public health issue, in France, polio was considered less pressing than other diseases. This article, based on original archives from the Pasteur and Mérieux institutes, analyses the polio control strategies and policies implemented by France from the mid-1950s to the end of the 1960s. The article examines the role of two key actors and institutions that mobilised the French health authorities against the disease: Pierre Lépine and the Institut Pasteur as well as Charles Mérieux and the Institut Mérieux. Lépine developed an effective injected polio vaccine which was first used before being supplemented with the oral polio vaccine. If the two main protagonists and their institutions worked together, they each implemented different actions and manoeuvres, at different times with the aim to raise awareness of the fight against the disease. The national and international relations of the key French actors were decisive in the development and production of the polio vaccines and their application. This work contributes to understanding processes of polio vaccines choice at the level of national institutions and analyses the political and scientific networks built in support of polio vaccination, to finally move towards compulsory vaccination. Ultimately, this study describes the historical processes by which this disease became conflated with a biotechnology of collective protection in France.