Prions (from "Proteinaceous" & "Infectious") were determined to be protein-based disease causing agents in 1982. In the last 20+ years, much research has been done to learn more about these unusual protein-based diseases, but many questions still remain.

Basically, prions are misfolded proteins. Proteins are made of chains of molecules calledamino acids, and they must be folded into specific shapes to function properly in the body (analogous to balloon animals or origami).

Other proteins in cells, called chaperones, guide the proper folding of proteins, and work to ensure that misfolded proteins don't get released into the cell. The normally folded protein, PrPc, is found in many animals, including humans, and is harmless.

PrPc seems to attach to the outside of cells, but what its normal function is remains unclear. Mice lacking the PrPc gene that codes for PrPc protein (so they lack all PrPc) seem healthy! However, when the PrPc protein is folded incorrectly, it is termed PrPsc, and this form is linked with many prion diseases.

It appears that incorrectly folded PrPSc influences the folding of other PrPcs, causing normal proteins to adopt the abnormal shape. There may be an unknown protein that matches the incorrectly folded protein with another PrPc to guide the abnormal folding. The abnormally folded proteins then accumulate in clumps and threads within the nervous system, leading to brain damage.

Prion diseases occur in humans and other animals, and these diseases are termed TSEs (Transmissible spongiform encephalopathies), or "diseases causing small holes in the brain that can be transmitted". Normally, the prion proteins vary enough between species that cross-species transmission is impossible. BSE (bovine spongiform encephalopathy) or Mad-cow disease was an alarming exception to this; where consumption of cow PrPSc resulted in human infection. It is thought that in the case of ingested prions, the PrPSc may get across the selective lining of the intestine by a "hitch-hiking" method used by some viruses, and then may accumulate in the immune/lymphatic system before traveling to the nervous system (including the brain).

TSEs do occur by methods other than exposure (through blood and food) to PrPSc. If an individual has a mutation in the gene coding for the PrPc protein, the proteins may misfold more readily, and this will cause an inherited disease. Also, it seems that there are cases of spontaneous PrPc mis-folding, without any genetic link, or exposure to misfolded proteins.

It has been recently found that yeast, which are eukaryotic organisms, more similar to animals than bacteria, prions may play a role in normal maintenance, and researchers are studying the proteins in yeast to gain a better idea of the function and possible treatment in animals. Scientists are also working on better & earlier TSE detection tests, especially for livestock, and on possibilities for vaccinations.