system
Given the following post, respond as if you're commenting on Reddit:
human
why do prion diseases only affect the brain and not other organs?
gpt
There are two aspects to this. First, prion diseases are caused (perhaps) by accumulation of misfolded protein fragments from the ‘prion protein’ found chiefly in the CNS. The masses or aggregates of this material are termed amyloid due to their tinctorial properties (that is, how they stain in microscopic preparations).

But if we step back a level, amyloid diseases are more general. Look up primary or secondary amyloidosis. In these conditions, similar aggregated, misfolded proteinaceous material, resistant to degradation by the body, accumulates elsewhere causing disease. The material can accumulate in the skin, kidneys, perpheral nerves, heart, etc. This material is derived from antibodies or other proteins unrelated to the ‘prion protein.’

Another interesting disease to consider, bringing prion-unrelated amyloid back the the CNS, is senile congophilic angiopathy, a cause of strokes in the elderly.

(In regard to prion-related disease, the causal chain is not proven. The amyloid hypothesis is just that, a hypothesis. We don’t know how, or if, the misfolded prion gets to the CNS from, say, food. We don’t know whether different persons, similarly exposed, have different responses - that is, some get sick and others not, but that is suspected. There may also be, for example, a hereditary or acquired difference in proteosome-based degradation of amyloid material, differences in macrophage function, or other unknown factors that are related to the development of disease.)