system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does prion disease work? Why does it happen on identical species only? (ex. cow eats cow, or human eats human)
gpt
If you’ve ever seen salt crystallise from salt water,  It works the same way. 

A tiny miss-folded protein from a “Mad cow” you’ve eaten moves through the blood brain barrier and into a gets into one of your neurones.  

Your neurone has lots of similar proteins, to the “Mad Cows “  protein. When these two proteins (yours and the cows) knock together it cause a change in your proteins it stacks together like lego with the miss folded cow protein and won’t come apart easily.

As more of your proteins hit this miss-folded stack they add and add to it.

As the stack grows your neurone detects a drop in that specific protein and produces more, which only grows the crystal stack of miss-folded proteins more.

This cycle continues until the neurone cant function and dies due to the crystallised protein within in it.

Once dead the neurone is broken down by the body causing pieces of miss-folded proteins to be spread to other neurones.

This is why it can take awhile before symptoms can appear, then degrade rapidly. The crystallisation is exponential.