Prion diseases https://www.cdc.gov/prions/index.html lurk within brains and nerve stems, with cows it became a big thing when the industry started making mega tonnes of offal waste mixed with grain feed pellets .. so all cows were being feed ground up other cows including brains.
Like most other diseases species -> same species is an easier vector than species -> different species.
Like most other diseases species -> same species is an easier vector than species -> different species.