Bacteria? Viruses? But what is a Prion?

Prions are abnormally folded proteins that are infectious, causing other normal proteins to misfold in the same way. The word "prion" is coined based on the phrase "proteinacious infectious particle".

Prions cause many transmissible spongiform encephalopathies, i.e. diseases that degenerate the brain, central nervous system, and their function. The commonly known Mad Cow Disease comes from prions in the central nervous system of cows. Once a misfolded protein forms, they act as a template for converting other normal proteins into prions. The prions accumulate and agglutinate, forming clumps that interfere with neurons. The transmission of signals in the nervous is interupted, resulting in nerve cell damage and death. Eventually, the brain gets a sponge-like appearance.

As prions are many times smaller than bacteria or viruses, it makes it hard for any immune system to detect their presence and destroy them. In this case, the antigen is simply a misfolded protein - something that an immune system would overlook. Usual techniques of sterilization such as UV light and radiation are ineffective against prions.

Fortunately, prions only form a small fraction of diseases.

Sources: 

1. http://www.news-medical.net/health/What-is-a-Prion.aspx

2. https://en.wikipedia.org/wiki/Prion

Images:

1. https://www.macalester.edu/academics/psychology/whathap/ubnrp/tse10/introduction.html

2. https://openi.nlm.nih.gov/imgs/512/342/2423184/

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