Discovery of the Plague Bacillus

French-Swiss bacteriologist Alexandre Yersin isolated the bacterium responsible for bubonic plague on June 23, 1894, while investigating a major outbreak in Hong Kong that had already killed roughly 100,000 people, working in a makeshift straw-hut laboratory after being denied access to hospital facilities reserved for his rival, Japanese bacteriologist Kitasato Shibasaburo. The organism, later named Yersinia pestis in Yersin's honor, was significant not only as the identified cause of plague but because Yersin also demonstrated that the same bacillus infected rodents as well as humans, pointing toward the animal reservoir and transmission route that made the disease so difficult to control. After returning to Paris, Yersin worked with colleagues to develop the first anti-plague serum and later established a laboratory in Nha Trang, in French Indochina, to manufacture it.