There's a really icky story on CNN that Mad Cow disease may have arisen from cows eating human remains.
The cowfood in question was imported as bone meal from south asia. The CNN article goes on: "In India and Pakistan, gathering large bones and carcasses from the land and from rivers has long been an important local trade for peasants," the scientists wrote. "Collectors encounter considerable quantities of human as well as animal remains as a result of religious customs." That is, they'll haul stuff out of the Ganges no questions asked.
It should be said a second hypothesis is that the disease came from the bones of sheep infected with scrapie. The authors of the Lancet paper point out that the first case of mad cow was in 1986, even though sheep stuff had been recylced for decades. There is also some evidence that cows don't get scrapie "via the oral route."
I know, I know, it's all just protein. Yuck.
UPDATES: A bit of policy comment on this from Crumb Trail.
Coincidentally, "chronic wasting disease," or CWD, another prion disease seen in elk and deer, seems not transmissible to humans.