[persedes]

In addition prions seem to contain disulfide bonds, which tend to make proteins even more stable. (RNAse for example has them and is notoriously hard to get rid off unless you treat it with essentially H2S).

I haven't found a source for that, but according to my bio teacher mad cow disease started, because they decided to lower the temperature to pretreat animal waste (such as bones and brain matter...) which then ended up in what cows get fed with. Wikipedia mentioned nowadays they simply leave out the bones and brain from the food.