Plague victim. Male plague victim with his arms crossed. Plague is a disease caused by the bacterium Yersinia (syn. Pasturella) pestis. It is transmitted by rat fleas and can occur in three forms: Bubonic, pneumonic and septicemic. The former was responsible for the Black Death which killed a quarter of the population of Europe. Although effectively wiped out from Europe it still causes occasional epidemics in less developed countries. Photographed in the early 20th century in San Fransisco, California, USA.