every major study on the transmission of mad cow to people have come up with the same conclusion: we don't know shit. I've heard that it could just be a spontaneously appearing genetic thing that appears in populations randomly, and just happens to resemble the cow disease.
as for cows: there is speculation that it comes from the feed. as you may or may not know, a large component in cow feed is COW. they take the parts of the cow that are left after butchering, and incorporate it with grain and other ingredients to make cow feed. there is speculation that bad stuff from cows that would normally die out with the cow that dies is getting re-introduced into the cow population, and builds up generation after generation until it comes to a head in the form of MCD.