Why did prejudice increase at the time of the black death? what factors account for the difference between the views of christian leaders and christian masses? why do aquinas and pope gregory oppose prejudicial action against jews?

1)  Prejudice increased at the time of the Black Death because it was an example of superstition made up by those who was really into religion. Many people believed that this disease was a punishment that God sent to sinful people. At that time, Jews were blamed and tortured, and some of them could not help themselves and lied that they poisoned the water wells, just to save their lives. 
2) The factor which account for the difference between the views of Christian leaders and Christian masses is that the Catholic Church is assured that the Mass is the same as the sacrifice which was offered by Jesus Christ. Christian leaders do not consider masses to be that significant as they are more pragmatic in that way.
3) Aquinas and Pope Gregory oppose prejudicial action against Jews because they have a belief that Jews deserved exactly the same treatment & equality as others including Christians & Muslims. Unlike those who supported all the prejudicial actions towards Jews, Pope Gregory and Aquinas had very progressive views which, of course, had influenced the decadence of prejudice.