A protein aggregate that can multiply itself, inducing the formation of new aggregates from individual copies of the protein it encounters. Prions have the potential to spread within the body and brain, and even from one organism to another—“infectiously,” like a virus. The first prions described were hardy aggregates of PrP, the prion protein. They are responsible for a set of rapid, fatal, and potentially transmissible neurodegenerative diseases including Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (“mad cow disease”). Many researchers now argue that protein aggregates in other neurodegenerative diseases, such as the Aβ and tau plaques of Alzheimer’s, have such similar properties that they also deserve to be called prions.