system
Given the following post, respond as if you're commenting on Reddit:
human
How did the black death of the 14th century spread?

If the plague was caused by the bacterium Yersinia Pestis, which was carried by fleas - how did it spread from human to human? Was it flea transmission? 

And as far as I'm aware there were 3 forms of the plague in 14thC Europe, was it all spread the same way?
gpt
It's super interesting actually!

So, the disease spread through fleas. Essentially, the bacteria infect the flees, and reproduce around their proboscis and "gum it up". When the flea goes to bite a human, it must "push" out it's blocked proboscis before it can take a blood meal, throwing the bacteria into the person's bloodstream. This is only half the story, however. 

Fleas stay on living bodies, so there are a couple of things that happen. If an infected person's body got too hot from fever, the fleas would move to a fresh host. Further, if a person died, the fleas would leave the cold corpse and seek a warmer, healthier body. This is why the disease spread quickly.