For me it’s because they have a component of “natural destructor” mixed with “indestructible evil”. A virus, a bacteria, an amoeba feel like mosquitoes, even if

For me it’s because they have a component of “natural destructor” mixed with “indestructible evil”. A virus, a bacteria, an amoeba feel like mosquitoes, even if deadly ones. Poisons and radiation feel like dangerous substances. They are all external and countable. Prions are your own bad design, a trivial key to a malfunction at the very core level, like an instant crystallization catalyst for your tissues, which you cannot undo. It’s not even your death that is terrific about prions, but the fact of the whole possibility of their existence - they feel just alien/enemy to our entire nature. I have similar feelings about SCP-009 (red water) and false vacuum.