Following up on my good colleague's post; it's a nomenclature thing. The infectious agent is called a prion (so there's no such thing as a "good" prion) but the protein it's made up of (PrP stands for "protease-resistant protein") is found in healthy people too, albeit in a different form.
Think of it like origami: the same square of paper can be a folded into a crane, or it can be folded into, say, an alligator. One's harmless, possibly even useful; the other one'll bite you in the a**.
Figuring out the how and why of protein folding is where most of the action is these days. And not just for diseases. Scientists would love to make synthetic polymers that can change shapes like proteins do. There's a very cool distributed computing project at Stanford called Folding@Home (modeled on Seti@Home) that's trying to sort it all out. If you've got a PS3 you can contribute your spare cycles to the project. You'll be rewarded with some wicked visualizations of the actual folding process in real-time (see screenshot above).
