The Science of The Callisto Incident
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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 PrPSc. And like "Ice Nine" from Vonnegut's book Cat's Cradle, PrPSc 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 PrPSc, 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 PrPSc 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...)