Swiss bacteriologist who discovered the bacillus that causes bubonic plague. During an outbreak in Hong Kong in 1894 Yersin isolated the plague bacillus from plague buboes. One year later he inoculated animals with anti-plague vaccine. Yersin also helped elucidate the role played by healthy human carriers in the transmission of epidemic diseases. Yersin and Pierre Roux (1853-1933) proved that the diphtheria bacillus produces a toxin that causes the signs of the disease.