They don’t “want” to exist, even to the minimal extent of a virus, they just happen.
Prions are in that sense no different - they are just less successful.
Viruses that don’t “want” reproduction fail to do so and disappear.
The essential idea is roughly "we end up with more of the things that replicate more". Despite the redundancy, there is a crucial idea in here. If we allow the use of the word "want" metaphorically then anything that replicates "wants" to reproduce including memes, crystals, prions and computer viruses.
Maybe you're just saying that a prion isn't as good at replicating as a virus? It seems like a difference of degree rather than kind.
They are qualitatively different.
A virus infects a cell, and critically, can quickly adapt (through random mutations) in an unthinking war between the various forces of the immune system and transmission mode efficacy; A prion does not evolve, it's a misfold of a single type of protein. If you want to compare it to a virus, it's like a virus that is incapable of replicating anything other than an exact copy - and can also only replicate itself using a particular cell (one type of protein).
Why the human body cannot effectively defend itself against particular propagating misfolded proteins is a mystery to me though... perhaps it's too low level to be identified by the immune system as something to attack, much like a poison.
That a virus can be a big and complicated structure does not mean that smaller and simpler structure is fundamentally different.
Viruses themselves also have many different modes of operation, with things like virophages, satellite virus and giant virus showing just how unclear the term "virus" is.
They are countable, but yes there are many ways, but that alone doesn't mean it's capable of meaningfully evolving. It's a search space multiple orders of magnitude smaller than a piece of DNA, so it's easy to see how a different successfully propagating misfold will be significantly less likely when starting from an existing propagating misfold.
If you are suggesting one propagating misfolded protein can, by chance, misfold into another propagating misfolded protein at a rate fast enough to become adaptive... then it's qualitatively the same as a virus, but I don't see this as likely given the significantly smaller search space.
The space of possible proteins - and thus prions - is not at all limited, neither figuratively nor literally. It is most certainly not "countable" without some Nobel-prize inducing discovery.
The only known human prion proteins seem to be a few orders of magnitude smaller than that though, and they don't appear to adapt.
Are you saying it's possible for prions to emerge from large proteins with the ability to adapt?
The viruses we see are common because they reproduce, it's not that they reproduce because they have some drive to become common.
Thus prions are less common because they aren't as successful in spreading (for some reason(s) I don't know either).
I don't believe they reproduce in any but the broadest sense either. Don't they just cause other proteins of the same type to misfold as well? It's not really a transfer of genetic material, it's a change in the emergent structure of the protein.
The prion's mode of operation isn't just "bump into stuff". It reaches the brain, causes other materials (DNA aptamers) to clump together, and causes other prions to be created.
That one happens to have a part that looks like genetic material and the other looks like a misfolded protein is merely a small implementation detail to me.
(Disclaimer: I, like everyone else I'm this thread, is not a qualified source of medical information.)
I've never heard of prions targeting certain tissues. I need to do some reading, as it's been a while since my last bio course. Do you have an article that explains how that works? In order to have that ability, they would have to be more complicated than I understand them to be.
The difference is that viruses have many more tools available in order to evolve and adapt. Prions, like crystal polymorphs, merely have the configuration space of the molecule. Viruses have the configuration space of dozens of proteins, and complexity can increase, so in theory there is no upper bound.
So I guess yes, they are all replicators that differ in extent by the amount of phase space available to their lifecycle. But the complexity jump between prions and viruses is huge, and prions are inherently bounded.
One main "behavior" difference between crystals, prions and viruses is that a crystal only forms by recruiting its equals when in high concentration - growth, but no replication.
Most virus is way more complicated than prions as you say, but a blue whale is also more complicated than a single-celled myxozoa and yet they are both animals. I do not think a comparison of complexity is meaningful in this debate.
One should also consider that there are countless proteins to misfold in any number of ways, so the prion space is not necessarily that limited. Virus do have the upper hand by using our well-oiled evolutionary machinery though.
I guess what I am saying is that either you are right and that can be extrapolated all the way up the chain to humanity. Or that you are wrong and you can apply free will all the way down the chain.
Today we have international animal shipping and trade, and global distribution. Prions are able to spread from exposure to shared pastures. A couple years of contamination used to be no big deal - today, if missed, it can spread all through the food supply.
In particular when you feed cows proteins made from dead cows it's making it more likely to create prion problems than when wild animals live in small herds in forests (and the ones that are sick are killed by predators or starvation before they can infect others more than likely).
So it doesn’t require it, it just increases the odds (and they don’t eat each other to spread it - it’s highly infectious and demonstratively infects if they share browsing/grazing land).
It doesn’t appear to spread between them and humans, but it definitely makes me nervous.
The chance of any infectious agent "jumping the species" due to cramped conditions or because of exposure of workers to contaminated animal parts in unclean slaughterhouses is immense - the current SARS-CoV-2 pandemic in my opinion is a warning signal of something much worse to come, and no matter what kind of agent it will be, we're pretty much screwed. Half the population or more of the world doesn't believe in vaccination, a quarter or more don't have access to vaccines or any kind of modern medicine at all, we are already running out on reserve antibiotics...