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, but may have identified a different bacterium and not the pathogen-causing plague. 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.