This is not the work of a virus, bacterium, or parasite. The culprit carries no DNA or RNA and may begin as one of the body’s own proteins. Known as a prion, this biological troublemaker can trigger normally shaped proteins to misfold, clump together, and gradually destroy the brain.

Prion diseases affect only about one in a million people worldwide. Yet their unusual behavior has made them important far beyond their rarity, offering researchers a window into how destructive proteins may also move through brains affected by Alzheimer’s and Parkinson’s disease.