A prion is an infectious misfolded protein that is known to cause several neurodegenerative diseases, such as Crutzfeld-Jacobs disease in humans and Bovine spongiform encehpalopathy ("mad cow disease") in cattle. While technically not living organisms, prions replicate themselves at the cost of host fitness and are considered parasitic.
The protein PrP is found throughout the body. Its exact function is unknown, but it is known to have a high affinity for copper ions and is believed to be essential for the nervous system to perform correctly. There are two isoforms of this protein; the first, called PrPC , is the normal functioning protein. Its pathogenic isoform is called PrPSc, and is a nonfunctional misfolded form of PrPC. It is highly resistant to denaturing by heating or pH changes; this is how it is able to survive and be transmitted though ingestion. It is hypothesized that contact between PrPC and PrPSc causes the normal protein to convert into the misfolded form, thus reducing the amount of functional protein in the body exponentially. In addition, the misfolded protein's twisted shape allows it to link into plaques that may cause harm to the host, likely contributing to the neurodegenerative properties of this type of infection.
A prion is a misfolded protein that can lead to scrapies in sheep, bovine spongiform encephalopathy (mad cow disease) in cattle, chronic wasting disease in white tailed deer, and Creutzfeldt-Jakob disease and Kuru in humans. This parasitic protein can be transmitted by the consumption of infected animal parts. Humans are exposed to this parasitic protein through the consumption of infected cattle, sheep brain, or human brain (Kuru).