Yep. A lot of it is personal biology and a lot is also chance. You can be shot in the head and survive with not much cognitive impairment, or you could hit your head against something and be unable to speak or feed yourself for the reset of your life. We are very pliable but it's sad that we're also still so fragile, at the level of the "mind" / "soul". Hopefully there'll eventually be a day where consciousness can be hosted in a much more safe way, for both biological and total immortality.

I don't think it's exactly correct to say "a prion" can destroy it. One single prion I don't think would cause significant damage in most cases, if there could somehow be only a single one. The issue is that it's believed a prion will gradually cause nearby proteins to misfold and also become prions, and then it probably starts recurring exponentially like a pathogen infection, and your brain physically deteriorates and falls apart with no way to stop or reverse it. A very similar deterioration process appears to happen with other disorders like Alzheimer's and the form of dementia discussed in this article, though the root cause still isn't understood.