How did Paul-Louis Simond explain the spread of plague?

In 1898, while working in India, the French researcher Paul-Louis Simond conducted experiments that provided a critical breakthrough in understanding how plague spreads. He demonstrated that the Oriental rat flea (Xenopsylla cheopis) was responsible for carrying the plague bacillus between rats.1

Building on observations that unusual rodent deaths often preceded human outbreaks, Simond’s work helped establish that these fleas, after losing their rodent hosts, would infest humans and transmit the disease through their bites. This discovery was pivotal in shifting public health efforts toward global rat-proofing measures and the use of insecticides to control the disease, moving away from earlier, incorrect theories that attributed the plague to "poisonous miasma" or vapors.21