There's basically one protein which does it (all the variants are just slightly different misfolds or misfolds of a slightly different version of the protein).

There's basically one protein which does it (all the variants are just slightly different misfolds or misfolds of a slightly different version of the protein). It's only present in the nervous system and doesn't generally spread from there nor does it very easily make its way there from the skin, gut, or lungs (though of course it does sometimes). It also seems that some people are immune or at least more resistant. This makes a wide spread of it extremely unlikely without some really specific circumstances (like cannibalism, either in humans or in animals, or a deliberate attempt at weaponisation).