Paul-Louis Simond (30 July 1858 – 3 March 1947) was a French physician, chief medical officer and biologist whose major contribution to science was his demonstration that the intermediates in the transmission of bubonic plague from rats to humans are the fleas Xenopsylla cheopis that dwell on infected rats.
Alexandre Emile Jean Yersin (22 September 1863 – 1 March 1943) was a Swiss-French physician and bacteriologist. He is remembered as the co-discoverer of the bacillus responsible for the bubonic plague or pest, which was later named in his honour: Yersinia pestis. Another bacteriologist, the Japanese physician Kitasato Shibasaburō, is often credited with independently identifying the bacterium a few days earlier. Yersin also demonstrated for the first time that the same bacillus was present in the rodent as well as in the human disease, thus underlining the possible means of transmission.