Prion - Wikipedia, the free encyclopedia: "Not all prions are dangerous; in fact, prion-like proteins are found naturally in many (perhaps all) plants and animals. Because of this, scientists reasoned that such proteins could give some sort of evolutionary advantage to their host. This was suggested to be the case in a type of fungus (Podospora anserina). Genetically compatible colonies of this fungus can merge together and share cellular contents such as nutrients and cytoplasm. A natural system of protective 'incompatibility' proteins exists to prevent promiscuous sharing between unrelated colonies. One such protein, called HET-S, adopts a prion-like form in order to function properly. The prion form of HET-S spreads rapidly throughout the cellular network of a colony and can convert the non-prion form of the protein to a prion state after compatible colonies have merged. However, when an incompatible colony tries to merge with a prion-containing colony, the prion causes the 'invader' cells to die, insuring that only related colonies obtain the benefit of sharing resources."