The fleas would try and suck the blood from the rat but that didn't work. So since they couldn't take the blood from the rats, the fleas would try and take the blood from humans and puke diseased rat blood in humans, infecting them with the Black Plauge.
The people of Britain were in panic mode due to the plague wasn't getting any better and since the Jewish people were in minority at that time and they were rich, so the serfs blamed them. The British people said the Jews were poisoning the wells and rivers.
Because of the decline in the peasant population, the serfs were gaining more and more power. But the french nobles were unwilling to bend to the new social standard, causing the serfs to riot.