The word prion is derived from the words proteinaceous infectious particle, and refers to the pathogen that causes transmissible spongiform encephalopathies, or TSE. TSE are neurodegenerative diseases that infect mammals. Cellular prion protein, or PrPC, is a normal protein mostly found on the surface of central nervous system cells. The purpose of this protein is not clearly known, but it is known that when these proteins fold in one particular incorrect way, they become prion protein scrapie, or PrPS. Prions are much smaller than viruses, and even through an electron microscope, only clusters of them are big enough to be seen. Prions make a unique pathogen, since, unlike bacteria, viruses, fungi, and other known biological pathogens, they lack nucleic acid. Since they don't have nucleic acid, they naturally resist procedures that destroy biological pathogens by breaking down nucleic acid. Also, since prions are an abnormal form of a normal protein rather than a recognizably foreign body, they don't stimulate an immune response in their hosts as other pathogens would.
How Prions Replicate
Since they don't have nucleic acid, prions can't reproduce. Instead, they replicate by stimulating normal PrPC to refold into PrPS. This refolding causes neural tissue degeneration and, eventually, death. The exact process by which the prion causes normal PrPC to convert to PrPS remains unknown.
There are many different diseases that are considered to be prion diseases. The first such disease discovered, for which the prion itself has been named, is scrapie. Scrapie is the TSE that infects sheep and goats, and has been known of since 1732, though of course it was not known as a prion or TSE until much more recently.
While prions do not seem to be able to move from one species to another, there are prion diseases for other mammals. Two such prion diseases that have made their way onto the scene more recently are bovine encephalopathy, or mad cow disease, and the chronic wasting disease that has been recently plaguing the American deer population.
The TSE that infects humans is known as Creutzfeldt-Jakob disease. The incubation period and the amount of time it takes for these diseases to impact the body will vary. It can be 5 years or it could be 20 years before enough prions have copied to create problems.
The long incubation period is helpful when developing drugs for these diseases. It means that the drugs do not need to get rid of all of the prions that are affecting the body. Instead, all that has to be accomplished with the drug is to slow down the growth of the prions so they cannot become so numerous as to affect the body. While this might not cure the infected individual entirely, it can, if successful, keep the effects suppressed.
The study of prion diseases is relatively new – only dating to the 1960s. It is still unknown exactly how PrPS coerces PrPC to refold, or even why it exists in the first place. The relatively recent discovery time, the odd nature of the pathogens, and the very small size all play a part in delaying a full understanding of these proteins.