Due to the shift in society more and more people were moving to areas where there was lots of civilization and the architectures were building the buildings so close together with no space it was home after home building after building and they did not have plumbing so they would go to the restroom in buckets and then just leave it on there streets and they would leave there trash on the street and dump it in there lakes and rats started to come and this is what led to the BLACK PLAGUE