There's a theory that it came from the feed that the infected cattle ate. A while ago, I read that the disease spread from sheep carcasses to living cows because the former were being fed to the latter.
Don't know if this is true and I'll be happy to learn more.
http://en.wikipedia.org/wiki/Bovine_Spongiform_Encephalopathy
As always, take a large grain of NaCl with a wiki anything. This one has some truth in it, especially on the spread of prions. But it makes no effort to separate the live/infectious from the not-live/contact transmission paradigms. Prions are like getting sick from touching a coin.
Either we are equal or we are not. Good people ought to be armed where they will, with wits and guns.