[water-data-dude]

You have one misfolded protein that “teaches” other proteins to misfold in the same way[1]. It’s very slow initially because you’re talking about individual proteins, and it takes a while for each prion to bump into a normal protein of the correct type. The immune system can’t do anything about them though, so once a protein is converted there’s permanently another prion. The conversion rate is very slow, but it never ever ratchets back - the number only increases. It’s a geometric progression.

Once symptoms show up there’s a TON of them, and it goes downhill fairly quickly. That’s why there’s such a long latency period.

[1]It’s like Ice-9, if you’ve read Cat’s Cradle.