Look at our immune system. Incredibly complex and clever, and able to keep us alive in the face of all sorts of pathogens. It exists because of this cat and mouse game, played over millions of years.
There's people in the highlands of PNG who regularly eat each other. Of course, many are thought to have died due to prion diseases. But now these tribespeople seem to have become largely immune to prion disease. Incredible.
Your comment on PNG, seems to ignore Kuru
From an evolutionary PoV a perfect defence is overkill - with two separate defences against prion diseases in that region it's only the rare variation that causes any issue - and that rarely occurs before a new generation is birthed - ie. 'perfect' from the PoV of the selfish genes.
The resistance came about via two separate "evolutionary upgrade"(s).
It appears to me that, like BSE (aka Mad Cow disease) it really depends on exposure.
There's a twofer that skittled the Fore, a ~1900 mutation that created a new form of infectious prion proteins, and a local variation that saw less uptake in the Fore of a resistant prion protein (alongside other resistant prion protein).
So, over the highlands region, there was general resistance thanks to several evolved variations, in one specific locale (the Fore) there was insufficient resistance to the mutation that hit a peak of 200 deaths / annum for about three years(?) in the late 50s.
I can't speak to "the literature", I just had a lot of conversations with the people on the ground (Mike Alpers, etc), on again / off again, since the mid 1960s.
Who said anything about a perfect defence? And since when was that the bar?
Just, you're doing it on your own.
Paper about Kuru and mutations referenced in comment here: https://news.ycombinator.com/item?id=49719102
That's not really enough to say "We have found the gene" - it's just really good data to warrant further investigation
Also, the incubation period of the disease is up to 50 odd years, have there been follow up studies?
Computers make copying, processing, manipulating, and disseminating information easy. That is a bad thing for some kinds of information.