yersinia pestis is a bacillus. it is a bacterium. it has been identified as the infectious agent of bubonic plague. this bacterium also causes other forms of plague- septicemic plague and pneumonic plague. these three forms of the plague have been responsible for a high death toll in many epidemics throughout human history. these diseases are believed to be the cause of the black death. because of the black death, about one third (one of three) people in europe died. this was between 1347 and 1353.