Prion proteins are those that can be mis-folded and cause harmful neurodegenerative diseases. A commonly known prion disease is one that affects cattle called Bovine Spongiform Encephalopathy, or "Mad Cow Disease". A newer form of this disease that affects humans is the Creutzfeldt-Jakob disease. Prion proteins are in the mammalian genome and are mostly expressed in the brain. In the body there exist two forms: a normal cellular protein and an isoform that is a pathogen. There is no difference in the chemical make-up of the two forms except that their conformations, folding is different. Research into the mechanisms in which a cellular prion protein becomes mis-folded remains unclear. This remains a problem because there is no effective way to combat the infections from such diseases. Unlike cancer or other pathogens that spread through the body by passing genetic material to the host cell and use it to generate more copies of themselves to infect more cells. Prion diseases cannot be targeted like cancer by finding cells that are rapidly multiplying, because the genome of the cell is not being affected only the proteins are. As a consequence the infectious agents can also be much smaller than viruses and bacteria.
The Prion Hypothesis
This idea that proteins can infect other proteins without the use of nucleic acids to transfer genetic material is contested. Part of proving the hypothesis is that the mechanism for the mis-folding is unclear. A search for possible cofactors that might facilitate the changes in conformations of the prion proteins could better explain and prove the hypothesis.