A prion is just a misfolded protine that has some adverse behavior that your body can't detect (there's a mechanism that if your body identifies a malformed protine, it will terminate the cell making it). Anyway, prions live in this small region in a Venn diagram whereits can't be detected, but can still replicate and cause harm.

We mostly think of prion diseases (like mad cow) affecting the brain, but I dont think prions are isolated to the brain... Prion deseases happen to involve the brain a lot because a misbehaving protine in your brain will have a lot more apparent effects