[yellowcat (OP)]

I have read that prion proteins are extremely stable and can remain pathogenic even after going through an autoclave or being treated with formaldehyde. They can be denatured by very high temperatures though.
How are prions so stable?
The only thing I can think of is that the pathogenic form of the prion protein is in effect already in a stable denatured state so it takes a not of energy to disrupt it any more.
Anyone have any thoughts on this?

[CliffordK]

I think proteins in general are more stable than one might otherwise expect. 

Say you cook a slab of meat, one expects most of the proteins in the steak or burger to remain intact. 

Bacteria and pathogenic eukaryotes would be damaged by liquids inside of the cells boiling and perhaps damaging of membranes and organelles with high temperatures.  I'm not sure about viruses, perhaps they also suffer membrane damage at high temperatures.

[evan_au]

Prions are polymers of a misfolded protein, forming a "pancake stack". This discourages chemicals like formaldehyde from getting between the proteins in the stack.

The shape of a protein is stabilised against heat by the proteins above and below it in the "stack", holding it in the misfolded shape.