Oh, and the incubation time can be extremely long. A burger you ate 5 years ago could've been infected but it could still be years before you begin showing signs. I remember reading a bit about some connection between prions and dementia in elderly individuals but I don't remember any of the details.

One sci-fi horror series had an episode where the plot was that some mass produced chickens were getting ill because the feed was made from other chickens and they eventually discovered that it involved a prion that was dangerous to humans as well. This was discovered after massive numbers of those chickens had been consumed in large quantities by society.

No. It is just that we do not have a cure or vaccine because it does not appear that often. And no, prions will not eradicate entire species because they not living under any definition. Therefore there is no direct evolution, if you are resistant against an outbreak, your are resistant against it. The prion cannot develop to overcome this resistance. Essentially, a prion disease is very like the computer virusses we have today: They do not change their source code, so fixing the issue in your code will disable them.