Alexandre Emile John Yersin (1863-1943), Swiss bacteriologist. After being educated at the universities of Marburg and Paris, Yersin became an assistant to Emile Roux at the Pasteur Institute, Paris. They collaborated in working on the toxins produced by the diphtheria bacterium. In 1894, whilst in Hong Kong, Yersin discovered the bacterium (Yersinia pestis) which caused bubonic plague (the 'black death'). The discovery was made simultaneously and independently by Kitasato Shibasaburo. In 1895 Yersin prepared the first antiplague serum. Yersin later founded two Pasteur Institutes in China and a medical school in Vietnam.