1) Poor public health. Medieval towns had no system of drains, sewers or rubbish collections. In such dirty conditions, rats lived and germs could grow.
2) Deaths. Estimates differ, but most historians believe that the Black Death killed half the population of Europe. In some places, eg the village of West Thickley in County Durham, it killed everybody. this led to the death-rate going bad in monasteries, where the monks stayed together and cared for each other. Some historians (Benedictow 2004) suggest that the wealthier classes were less affected due to their wealth enabling them to flee from outbreaks. led to the plague exceedingly spreading.
