Prions are a totally unique
disease-causing agent. They are known to be responsible for
bovine spongiform encephalopathy (BSE or
mad cow disease),
Creutzfelt Jacob Disease (CJD),
scrapie, and
Kuru. They are so unusual because they contain no
genetic material in the form of
DNA or
RNA like a
virus or
bacteria. They are composed entirely of
protein, which was previously thought to be incapable of causing an
infectious disease.
It seems that the proteins responsible for these diseases are normal proteins present in the brains of sheep, cattle, and humans. In their normal state, they are folded in a certain
conformation, consisting of mainly
alpha-
helical domains. In the variant, disease causing form, they seem to be folded in a different conformation, mostly
beta-
pleated-
sheets. It seems that this switch in structure can be transmitted, as though the variant (disease causing form) catalyzes the change in otherwise normal proteins.
The disease has been proven transmissible through cannibalism (eating brains in particular for Kuru), inoculation, and through genetic predisposition (most common in CJD, due to a mutation in the protein chain that makes it more likely to switch conformation). It has been shown that brain extract from BSE-infected cattle is capable of inducing CJD in chimpanzees, suggesting that the same could happen for humans.
These diseases all affect the brain, causing
ataxia, progessive
dementia, and ultimately, death in their respective species.