One of the reasons why the black death was so deadly was likely: that the yersinia pestis bacterium of the middle ages was particularly virulent. that people were afraid to visit doctors for treatment. that the great famine had weakened many people's immune systems. that most people contracted scarlet fever along with yersinia pestis. that it was regularly used as a form of biological warfare.

The Black Death is considered as the deadliest pandemic in history. There were four main forms of this plague namely: bubonic, pneumonic, septicemic and enteric plague. It wiped out around 200 million people from  Europe and Asia. It was caused by yersinia pestits, a type of bacteria transmitted through flea bites or inhalation. One of the main reasons why the plague spread quickly was the terrible health condition of the city dwellers due to poverty and malnutrition. The great famine of 1315 struck many parts of Europe and it lasted for almost seven years. The famine ultimately weakened the immune system of the dwellers in the cities.

In 911, a group of vikings lead by rollo besieged paris and chartres. after a victory near chartres on 26 august, charles decided to negotiate with rollo. the talks were led by hervé, the archbishop of reims. this resulted in the treaty of saint-clair-sur-epte, which granted rollo and his soldiers all the land between the river epte and the sea. it also granted him brittany, "for his livelihood". at the time, brittany was an independent land which france had not been able to conquer. in exchange, rollo promised the king his loyalty and military assistance when needed. also rollo also agreed to be baptised and to marry gisela, the king's daughter. the territory given rollo is about the same as upper normandy. this would grow to become normandy under rollo's descendants. - wikepedia