Most of the time, mental illness and neurodegenerative diseases are confined to mind and body of the afflicted… but there is an exception to almost every rule. The exception to this rule is called the prion, but most of you may know it as “what causes mad cow.”
A prion is an infectious particle that, unlike all other forms of infection, doesn’t have any DNA or RNA. Prions, which were only first discovered in the 1980s, are misfolded proteins that “persuade” (induce) other similar proteins to take the wrong form. These misfolded proteins sort of stack together, occasionally breaking apart and forming new chains, and their numbers increase exponentially once even a single prion has entered the body. Prions are completely untreatable and frequently lethal.
In humans, prions seem to always cause neurodegerative disease. They seem to have a genetic basis, with 10-15% of prion infections connected to the PRNP gene, with the most common manifestations being Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), and fatal familial insomnia (FFI). The remaining 85% of cases are likely contracted through food, needles, or otherwise unexplained mutations or malfunctions in protein folding.
The NIH explains the non-genetic cases by saying,
Acquired prion disease results from exposure to PrPSc from an outside source. For example, variant Creutzfeldt-Jakob disease (vCJD) is a type of acquired prion disease in humans that results from eating beef products containing PrPSc from cattle with prion disease. In cows, this form of the disease is known as bovine spongiform encephalopathy (BSE) or, more commonly, “mad cow disease.”
You would think that we could remove the risk of catching prions from contaminated meat by cooking it, or hope that the proteins would be denatured in our stomach acid, but this is actually wrong. Ironically, the misfolded form of prions gives them increased resistance to chemical and temperature related denaturation. Despite not having any DNA, prions appear to also be able to adapt and evolve: successful particles “reproduce” more effectively than unsuccessful prions.
Prions truly are the incurable crazy you can catch, but your odds of catching them sink (much like your ecological footprint) if you reduce your intake of meat. If you want, you can also get your genes sequenced to figure out how likely you are to “catch” genetically triggered prion disease. As of right now, there are no methods for either the effective prevention of catching prions from meat or for treating the disease once you have it.