1899 - 1800United States of America

L. Simond was the first to realize that manipulating a rat that had recently died could be extremely dangerous whereas after a time lapse of several hours the same dead rat presented no risks to man. He was also the first to detect an insect bite as being responsible for the lesions he had observed at the beginning of bubonic plague outbreaks and called “precocious plyctenas”. After having verified the presence of the bacillus in fleas of dying rats, on second June 1898, Simond carried out his principle experiment on the transmission of plague to rats by fleas. From then onwards, disinfection and pest control are plague prophylaxis.

The primary vectors for transmission of the disease from rats to humans were the Oriental or Indian rat flea, Xenopsylla cheopis, and the Northern or European rat flea, Nosopsyllus fasciatus. The human flea, Pulex irritans, and the dog and cat fleas, Ctenocephalides canis and felis, were secondary vectors. In the pandemics, the infected fleas were able to spread the plague over long distances as they were carried by rats and by humans travelling along trade routes at sea and overland, and also by infesting rice and wheat grain, clothing, and trade merchandise. When infected, the proventriculus of the flea becomes blocked by a mass of bacteria. The flea continues to feed, biting with increasing frequency and agitation, and in an attempt to relieve the obstruction the flea regurgitates the accumulated blood together with a mass of Yersinia pestis bacilli directly into the bloodstream of the host. The fleas multiply prolifically on their host and when the host dies they leave immediately, infesting new hosts and thus creating the foundations for an epidemic.



  • Mollaret HH. “The discovery by Paul-Louis Simond of the role of the flea in the transmission of the plague”.
  • Ayyadurai, S., F. Sebbane, D. Raoult, and M. Drancourt. 2010. Body lice, yersinia pestis orientalis, and black death. Emerging infectious diseases 16:892–3.
  • Frith J. “The history of Plague – Part 1. The Three Great Pandemics”. History, (Vol. 20), Number 2.