It's not yet freely available online, but the Scientific American Origins issue has a nice short summary of the origins of bovine spongiform encephalopathy, aka "Mad Cow disease".
Turns out Mad Cow is now thought to be related to President Jimmy Carter's sweater. The first (prodromic) energy crisis led President Carter to wear a sweater in the chillier White House, and changed animal rendering from boiling to centrifugal separation. The centrifuges were kind to prions, so millions of cattle, uncounted small animals, and about 200 humans died a miserable death.
The true root cause, however, was cannibalism. Rendered cows were largely fed to non-rendered cows.
Eating one's own species turns out to be quite unhealthy - despite the compatible food stock. Prions are therefore a de facto form of species-specific poison, and they would contribute to natural selection against conspecific cannibalism.
Prions are presumably a relatively small contributor to the contra-cannibalism trend, but this is an odd upside to the otherwise blameworthy prion.
Update: The 200 dead number will grow. Sadly.--
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