The plague has accompanied humanity for at least 5000 years. The underlying pathogen, the bacterium Yersinia pestis, was only described in 1894 by Alexandre E. J. Yersin. The plague can present itself as bubonic plague, pneumonic plague, and plague sepsis. While the mortality rate from an untreated bubonic plague is about 60 %, pneumonic plague and plague sepsis are virtually always fatal if untreated.
Three pandemics, in the 6th, 14th and 19th century, together have cost the lives of well over 100 million people.
The plague has lost much of its former terror – yet it has not been defeated. Between 2010 and 2015, the WHO reported over 3000 cases of plague, of which about 18 % were fatal.