Until recently science thought all infectious diseases in animals and humans were caused by living organisms, namely bacteria, viruses and fungi. In other words, infectious disease was caused by a living entity that contained DNA or at the very least RNA.
Hence our treatments for farm animals and humans are designed to kill the offending organism. But you can’t kill that which is not alive. Prions, basically malformed proteins, are not alive by any standard. They lack DNA yet they self-replicate and spread, hence their infectious nature. Now scientists are struggling to find a way to treat diseases caused by killer dead things.
You could think of them as zombies if you have a fondness for science fiction. But make no mistake prions are not fiction.