Alexandre Emile Jean Yersin (22 September 1863 – 1 March 1943) was dual national Switzerland and French people physician and bacteriology. He is remembered as the co-discoverer of the Bacilli responsible for the bubonic plague or pest, which was later named in his honour: Yersinia pestis. Another bacteriologist, 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.
In order to be able to practice medicine in France, Yersin applied for and obtained French nationality in 1888. Soon afterwards (1890), he left for French Indochina (current Vietnam, Laos and Cambodia) in Southeast Asia as a physician for the Messageries Maritimes company, on the Saigon-Manila line and then on the Saigon-Haiphong line. He participated in one of the Auguste Pavie missions. In 1894 Yersin was sent by request of the French government and the Pasteur Institute to Hong Kong, to investigate the plague happening there.
There, in a small hut (according to Plague by Wendy Orent) since he was denied access to British hospitals at his arrival, he made his greatest discovery: that of the pathogen which causes the disease. Dr Kitasato Shibasaburō, also in Hong Kong, had identified a bacterium several days earlier. There is controversy whether this was the same pneumococci or a mix of the two. Because Kitasato's initial reports were vague and somewhat contradictory, some give Yersin sole credit for the discovery. However, a thorough analysis of the morphology of the organism discovered by Kitasato has determined that "we are confident that Kitasato had examined the plague bacillus in Hong Kong in late June and early July 1894", only days after Yersin announced his own discovery on 20 June. Therefore, Kitasato "should not be denied this credit". The plague bacillus develops better at lower temperatures, so Yersin's less well-equipped lab turned out to be an advantage in the race with Kitasato, who used an incubator. Therefore, although at first named “Kitasato-Yersin bacillus” by the scientific community, the microbe will later assume only the latter's name because of the one identified by Kitasato, a type of streptococcus, cannot be found in the lymphatic glands. Yersin was also able to demonstrate 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. This important discovery was communicated to the French Academy of Sciences in the same year, by his colleague Emile Duclaux, in a classic paper titled "La peste bubonique à Hong-Kong".
From 1895 to 1897, Yersin further pursued his studies on the bubonic plague. In 1895 he returned to the Institute Pasteur in Paris and with Émile Roux, Albert Calmette and Amédée Borrel, prepared the first anti-plague Antiserum. In the same year, he returned to Indochina, where he installed a small laboratory at Nha Trang to manufacture the serum (in 1905 this laboratory became a branch of the Pasteur Institute). Yersin tried the serum received from Paris in Guangzhou and Amoy, in 1896, and in Bombay, India, in 1897, with disappointing results. Having decided to stay in his country of adoption, he participated actively in the creation of the Medical School of Ha Noi in 1902, and was its first director, until 1904.
Yersin tried his hand at agriculture and was a pioneer in the cultivation of ( Hevea brasiliensis) imported from Brazil into Indochina. For this purpose, he obtained in 1897 a concession from the government to establish an agricultural station at Suoi Dau. He opened a new station at Hon Ba in 1915, where he tried to acclimatize the quinine tree ( Cinchona ledgeriana), which was imported from the Andes in South America by the Spaniards, and which produced the first known effective remedy for preventing and treating malaria (a disease which prevails in Southeast Asia to this day).
Alexandre Yersin is well remembered in Vietnam, where he was affectionately called Ông Năm (Mr. Nam/Fifth) by the people.
On 8 January 1902, Yersin was accredited to be the first Headmaster of Hanoi Medical University by the Governor-General of French Indochina, Paul Doumer. Following the country's independence, streets named in his honor kept their designation and his tomb in Suoi Dau was graced by a pagoda where rites are performed in his worship. His house in Nha Trang is now the Yersin Museum, and the epitaph on his tombstone describes him as a "Benefactor and humanist, venerated by the Vietnamese people". In Ha Noi, a French lycée has his name. A private university founded in 2004 in Da Lat was named "Yersin University" in his honour.
In 1934 he was nominated honorary director of Pasteur Institute and a member of its Board of Administration. He died during World War II at his home in Nha Trang, in 1943.
While in Hong Kong, Yersin was helped in his research by an Italian priest of the PIME order, Bernardo Viganò (1837–1901). He provided cadavers and assisted him with his quest to find a remedy for the plague.