In a scattershot pattern that now extends from coast to coast, continental US states have been announcing new hotspots of chronic wasting disease (CWD).
The contagious and always-fatal neurodegenerative disorder infects the cervid family that includes deer, elk, moose and, in higher latitudes, reindeer. There is no vaccine or treatment.
Self-replicating protein fibers growing like plant roots through brain tissue? The only thing I can think of that would be worse is that fungus that infects an ant’s brain and makes it go crazy.