[Alexandre Yersin (1863-1943), explorer and a founding scientist of the Pasteur Institutes]

2013 was the year to celebrate Yersin: the 150th anniversary of his birth and the 70th anniversary of his death. Beyond the images usually attached to the memory of this doctor who discovered the bubonic plague bacillus (in Hong Kong in 1894), the author seeks to introduce Yersin, the man, as an explorer curious about his environment rather than a scientist concerned with honors and public recognition. Alexandre Yersin is an atypical figure in the universe of Pasteur, his collaborators, students, and followers. Although he began his career working with Louis Pasteur following the development of the vaccine against rabies, in 1885, the call of the sea led him to quit the laboratory on rue Ulm to, he said, "explore new lands". He worked for the Messageries maritimes merchant shipping company. In Saigon, he met Albert Calmette, who convinced him to join the newly created Colonial Army Medical Corps. In 1892 in Nha-Trang, Yersin set up a bacteriology laboratory in a straw hut; it subsequently became the first Pasteur Institute in Indochina, the starting point of a network of research laboratories. During the bubonic plague epidemic that raged in Hong Kong, Yersin succeeded in isolating its causal agent, surprising even himself by the ease with which he did so. He was 30 years old then, but what could have been the start of a prestigious career, crowned with honors, was spent instead at the service of the local populations. His exploration of the Vietnam highlands gave Yersin the occasion to cultivate and reveal a prodigious eclecticism and his profound humanism. He led three explorations in unknown regions of Annam and contributed to the development of this country by his social, educational, medical, and economic approach, entirely dedicated to aiding the indigenous populations. Yersin never left Vietnam again. He worked as an astronomer and agronomist (introducing the cultivation of cinchona (source of quinine) and rubber trees in the country) - always close to the population. He is buried at Nha-Trang; the Vietnamese continue to honor his memory fervently.