Viruses, viroids and prions

Reference: NCERT Class XI, page 21.

Bovine spongiform encephalopathy (mad cow disease) in cattle and Creutzfeldt-Jakob disease in humans are caused by prions, which are abnormally folded proteins.

Prions are infectious agents made only of abnormally folded protein; they contain no nucleic acid.
They cause neurological diseases such as bovine spongiform encephalopathy (mad cow disease) in cattle, scrapie in sheep, and kuru and Creutzfeldt–Jakob disease (CJD) in humans.
Viroids are the opposite: free, infectious RNA of low molecular weight without any protein coat (e.g., potato spindle tuber disease).
Satellite viruses and gemini viruses contain nucleic acid along with protein.
Hence infectious proteins are present in prions.

In 1971 T.O. Diener discovered a new infectious agent, smaller than viruses, that caused potato spindle tuber disease.
It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.
The RNA of the viroid is of low molecular weight.
Bacteriophages are viruses that infect bacteria (discovered by Twort and d'Herelle), and infectious proteins are prions (Prusiner).
Hence Diener discovered a free infectious RNA.

Mad-cow disease is bovine spongiform encephalopathy (BSE), a fatal disease of the nervous system of cattle.
It is caused by a prion, an infectious agent that consists only of an abnormally folded protein and contains no nucleic acid.
The abnormal prion protein makes the normal proteins of the brain cells change into the abnormal form, and their accumulation destroys the nerve cells, giving the brain a spongy appearance.
Its analogous variant in humans is Creutzfeldt-Jakob disease (CJD); scrapie of sheep is another prion disease.
It is not caused by a bacterium, a worm or a virus.
So the causative agent of mad-cow disease is a prion.