A prion can't just fold any old protein into itself; the vicitimized protein has to be made up of the same amino acid sequence as the prion, just folded in a different manner. So what determines how proteins fold? That's a tough question; even the most powerful computer simulations are just beginning to get a predictive handle on the problem. So although the mechanism isn't clear, the effect can be a chain-reaction: once a few prions start their work, the effect can snowball.