The hypothesis for many prion diseases is that of "seeding" whereby a small number (or maybe even a single) prion protein acts as a template to induce misfolding of other proteins with similar sequence but native folds [1,2]. This causes a chain reaction in the proliferation of the prion protein inside the cell, and then in other cells as prion proteins are transported/secreted to the surrounding tissue.
There is an advantage in thinking about prions, and proteins more generally, as crystals rather than biological systems. A vat of fluid can sit for decades without ever forming a crystal. But then the addition of a single seed can give rise to an infinitely large, growing, structure.