Alexandre Emile Jean Yersin 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.

Yersinia pestis is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both Yersinia enterocolitica and Yersinia pseudotuberculosis, the pathogen from which Y. pestis evolved and responsible for the Far East scarlet-like fever. It is a facultative anaerobic organism that can infect humans via the Oriental rat flea. It causes the disease plague, which caused the Plague of Justinian and the Black Death, the deadliest pandemic in recorded history. Plague takes three main forms: pneumonic, septicemic, and bubonic. Yersinia pestis is a parasite of its host, the rat flea, which is also a parasite of rats, hence Y. pestis is a hyperparasite.