* Do we have a good grasp on the number of prions in the world? My recollection from when MadCow became a big thing in the late 90s was that prions weren't even looked for and were hard to find as a general rule, but that was 20+ years ago. Might there be many common-but-harmless prions that we haven't even noticed?
* WHY is brain consumption (both in humans and in animals) such a source of prions? What makes that situation more likely to create prions?
People talk about Mad Cow, Kuru, Creutzfeldt-Jacobs, Chronic Wasting Disease, Scrapie, and a dozen other diseases like they are not all manifestations of the same underlying problem with PRNP gene variants.
From what I remember on the topic, we do not have a good grasp on prions disease. There is a good chance that some other diseases in history may have actually been misdiagnosed prions disease. It can present as a form of dementia and the incubation period can be so long that many people are middle-aged before symptoms occur.
Tissue of the brain and spine are better for Prions propagation. Prions cause other proteins to misfold themselves. The proteins they affect are related to brain matter.