The black rats are what started the plague by carrying the fleas that had a disease. Then when the villagers were traveling the rats transferred the fleas to the people.
The Christians believed that the Jewish people started the plague. So they burnt all the Jewish people that didn't want to convert to Christianity.
After many of the peasants were killed from the plague the ones left behind had to work harder to keep up with all land that other people owned. So the peasants demanded that get more pay/things that they wanted in return for working harder.