In 1897, when Alexandre Yersin was transferred by the Pasteur Institute to a post in Vietnam, his position in Bombay was filled by Simond, who was to test the efficacy of an experimental antiserum against the outbreak of plague in that city. The following year, Simond was in Karachi where, despite limited resources, he was able to demonstrate that fleas transmit the bacterium Yersinia pestis, the agent causing bubonic plague, from rat to rat, and from rat to human. His ingenious experiments are described in his report to the Pasteur Institute on the spread of plague ("La propagation de la peste"), which was published in 1898 by Pierre Paul Emile Roux in the "Annales de l'Institut Pasteur". For this work Simond was awarded the Barbier prize from the French Academy of Medicine. His findings were not initially accepted by the scientific community, but were validated later by others and by 1907 his conclusions were accepted as scientific fact.