Most people have heard of Mad Cow disease; fewer people know that the disease agent is neither a bacteria nor a virus, but rather a prion, something that's not even alive! Mad Cow is just one of a family of diseases; Creutzfeldt-Jakob Disease, Fatal Familial Insomnia, and Kuru are all human versions.

There's a protein in human and animal brains that has been come to be known as the Prion Protein, or "PrP". PrP is probably responsible for transporting or storing copper. [Kretzschmar, Nature, Dec. 1997] But, in a discovery that won the 1997 Nobel Prize in Medicine, sometimes a "rogue" version of PrP can appear that is identical in make-up, but with the amino acids folded into a different shape (from a shape with high alpha-helix content to one with high beta-sheet content). This abnormal protein is called PrP^Sc. And like "Ice Nine" from Vonnegut's book Cat's Cradle, PrP^Sc can fold normal PrP into copies of itself -- or reproduce! [G.C. Telling, Science 274, 2079 (1996)]. This can create a chain-reaction in the brain, turning all the healthy PrP into PrP^Sc, and in the process turning the brain into a sponge and eventually killing the person. It also affects the nervous system, spleen, and pancreas.

Because this is not a human-specific protein, the disease can transfer from animals to people much more readily than can bacterial or viral diseases, which are species-specific. Thus the practice of grinding up the brains of "mad cows" and feeding them to other cows allowed PrP^Sc to get into the meat of the cows, which were then eaten by humans. (Eating brains is another way to get the disease, so watch your diet...)