How do prions transmit their conformation to other proteins?
It is thought that infectious prions exist as clusters forming a crystalline structure. When a protein with the same primary structure is encountered but with a different tertiary structure, the normal protein undergoes a conformational change in order to integrate into the cluster. Presumably there are molecular forces involved that induce the conformational change.

http://www.rsc.org/chemistryworld/Issues/2005/October/prions.asp