Some pathogens ( such as the bacterium Yersinia pestis which may have caused the Black Plague , the Variola virus , and the malarial protozoa ) have been responsible for massive numbers of casualties . Some pathogens ( such as the bacterium Yersinia pestis which may have caused the Black Plague , the Variola virus , and the Malaria protozoa ) have been responsible for massive numbers of casualties and have had numerous effects on afflicted groups .