The Black Death, which is believed to have originated near China or the Mongolian steppes, was a catastrophic outbreak of plague that resulted in an estimated 25 million deaths across Europe alone. Caused by bacteria transmitted by rat-borne fleas, the disease took hold in both urban and rural areas and was spread most effectively aboard ships, which could bypass natural barriers. The disease was first introduced to Europe by Mongols, who attacked the Crimea in 1346 and from there it was brought west by traders who disembarked at Sicily and spread steadily northwards. A lack of scientific understanding mixed with widespread superstition at the time provided ideal conditions for the spread of the disease. In the case of mountainous areas, for example around Milan, and remote areas with reduced outside trade, such as Poland, the Black Death’s impact was significantly less prevalent. Casimir the Great also quarantined the borders of Poland, which greatly reduced infection from outsiders.