Why was the black death so devastating?

The black death resulted in an enormous death toll. It is estimated between a third and half the population died in countries where the black death hit. I am curious why the black death was so devastating and the were the following factors in causing such a high death toll :-

*If people caught bubonic plague from flea bites and the fleas were carried by rats, were there factors such as poor hygiene and sanitation which enabled rat populations to flourish and people were in close contact with rats.

*In a documentary about the black death, it was suggested that people were malnourished due to poor harvests which weakened the immune system and made people more suspectable to diseases such as bubonic plague. How credible is this explanation? If you were bit by a flea, was the pathogen so devastating a healthy immune system would have made no difference?

*If a pathogen is introduced by bites from fleas or animals, does this make a pathogen more dangerous due to how bites make a pathogen enter the body? Does this explain why diseases such as Ebola and Rabies are so deadly?