[John Valentine]

Prions are very mysterious things, and I’m not sure there is real agreement as to whether they even exist.

It is clear that there are malformed proteins in Mad Cow, but whether they were made that way by “prions” or not... well maybe the jury is still out on that question.

If they don’t really exist, than that would explain why it’s so hard to destroy them.

[newguy357]

It is clear that there are malformed proteins in Mad Cow, but whether they were made that way by “prions” or not...

Those malformed proteins are the prions. They then act as a misshapen cookie-cutter for new proteins of the same kind being created by the cells in question. Theoretically.

[mysterio]

There's a general agreement that they exist, and some characterization.

From NIH :

Prions enter brain cells and there convert the normal cell protein PrPC to the prion form of the protein, called PrPSC. When normal cell proteins transform into prions, amino acids that are folded tightly into alpha helical structures relax into looser beta sheets. More and more PrPC molecules transform into PrPSC molecules, until eventually prions completely clog the infected brain cells. The cells misfire, work poorly, or don't work at all. In mad cow disease, for example, with their brain cells running amuck, the mad cows wobble and stagger and appear fearful--their "madness" is craziness, not anger. Sheep and goats with the disease scrapie, which is like mad cow disease, become so uncomfortable and itchy that they frantically rub up against anything they can, finally scraping off--hence, the name of the disease--most of their wool and hair (2).

Here are some articles from PBS, written to be more easily understandable, with some arguments for and against prions : link