Viruses are genetic material encased in protein. They self-replicate and cause disease.
Prions are just proteins. They self-replicate and cause disease.
So complete the pattern: Plasmids are just genetic material. They self-replicate, but do they cause disease? Not that I can find, anyway.
Are there any diseases caused by plasmids?
Not sure what you mean by “self-replicate”. Plasmids are small, circular pieces of DNA separate from the bacterial chromosome. The require the cell’s DNA replication machinery to make copies, though they replicate independent of the cell cycle, unlike the main chromosome. They don’t replicate themselves, though. Viruses also require a host cell’s replication machinery to make copies of themselves. Perhaps you can define what you mean by self replicate.
However, many plasmids carry antibiotic resistance genes, and can be transferred between bacterial cells (spread through a population), so they can have a definite impact on health.
Rather than plasmids, I think the piece that you’re really missing is transposable elements (which are just segments of DNA). These are the origin for well over half of the genetic material in the human genome.
They originate as “selfish genetic elements” - originally present not because they encode a fitness phenotype, but simply because they are segments of DNA that are good at getting themselves copied. Some of them excise themselves in one place and reinsert in another (cut and paste), others insert new copies of themselves (copy and paste). These excision and insertion events can interfere with genes to cause disease. But they also provide a substrate of mutation upon which natural selection can act, and there are many examples of functional parts of our genome that evolved from transposable elements. A notable example is the RAG genes that generate diversity in the vertebrate immune system.
Some transposable elements are similar to viruses. Most Endogenous retroviruses (ERVs) exist now only as traces in the genetic material, but have an interesting evolutionary relationship with true retroviruses (i.e. genetic material which can package itself to disperse outside the organism). It may be that they originated as viruses; but it may also be that viruses originated from them, in other words, a kind of transposable element that originally existed only as part of the genetic material evolved the ability to package itself into protein and escape the host to infect other organisms.
Viruses and prions do not self-replicate.
Viruses usurp the replication machinery from a living cell. That is, they cause something else to replicate them.
Prions cause pre-existing proteins to mis-fold. We don’t say ice crystals self-replicate just because new ice crystals form around them in under some conditions.
OK, I should have known I’d get into this.
I understand how viruses and prions work. Self-replication by hijacking or inducing mis-folding, respectively, might not be what biologists have in mind for self-replication, but the end result is still more virus particles and prions in the world.
This is, ultimately, a semantic argument, and one that’s distressingly good at making more instances of itself by hijacking threads. 
Riemann: That’s very interesting.
I disagree that it’s semantic, but it’s probably not relevant.
The main difference between prions/viruses and plasmids, is that prions/viruses hijack the human host cell, whereas plasmids work within the bacterial cell. There is less opportunity to cause human illness under those circumstances.
Just say “replication” instead of “self-replication”. Viruses and prions replicate, but not by themselves.
Although since each of us maintains a microfloral ecosystem on and within ourselves, there are probably ways for plasmids to indirectly cause human disease.
Absolutely. Bacterial gene expression in the gut directly affects the gene expression of human cells lining the gut. However it’s likely that any plasmid affect will be more indirect tan direct. Antibiotic resistance if often encoded on plasmids, as I mentioned upthread, and is another impact.
Another way viruses can have an effect is through bacteria they infect. A virus can insert its DNA into its bacterial host, causing the bacteria to secrete a toxin, making the bacteria a human pathogen.
To go all semantic on everyone again, I never limited this to human diseases.
This is very interesting.
Maybe does not matter for your question but, no it isnt semantic
Amoeba = Self Replicate
Virus = Replication using the hosts cell replication mechanisms
Unless you’re considering bacterial diseases, you can extrapolate what I said about human cells to just about any other organism.
Again, in general, prions and viruses infect host cells and inflict damage. Plasmids are a normal part of bacterial cell biology and are not damaging to the cell.
Given how evolution works, there must be some plasmids that have little or no benefit but are passed on to other bacteria because the plasmid “encourages” the bacterium to replicate it and exchange it. Or, stated another way, there’s likely a blurry line between plasmids and virus in some instances. Just like the line between bacterial DNA and virus DNA is sometimes blurred.
The term you’re looking for is viroid: Viroid - Wikipedia A short piece of infectious genetic material. And yes they cause disease – in plants and fungi.
Generally, naked genetic material can’t survive inside an animal with even a barely functional immune system. Heck, animal cell just simply don’t work that way – we’re trying to advance genetic medicine, but we really have to package the genetic material and trick the animal cell not only into internalizing it, and not degrading it, but actually move it to the cell’s genetic machinery. And it often just doesn’t work.
Oops. I’m wrong. I forgot Hepatitis D, from the article linked above.
[EDIT]
No the article is wrong. Even Hepatitis D has a protein coat.
I worked with plasmids some in the lab I used to volunteer in.
Look into “plasmid addiction”. Addiction module - Wikipedia
I’d call that a disease caused by a plasmid. If the bacterium dares to shed the plasmid, it will die of a lingering poison for which the plasmid produced the short-lived antidote. Thus, only bacteria which maintain the plasmid continue to exist and reproduce.
In a fairly real sense, plasmids do sort of exist as distinct creatures. They’re responsible for some conjugation between bacteria as well, to infect others. That they usually present benefits to the host bacteria benefits the plasmid’s survival as well, of course, because the bacterium is their environment. I’ve heard mention of plasmid-related diseases in some plants, but have no specific memories to relate to you.
I seem to be unable to edit my previous post, or at least can’t see the buttons, so I’ll have to double post.
Since natural selection operates on all things, from rocks to ideas in human minds, the difference between cellular product and independent entity can be sketchy. Take a look at Origin and evolution of polydnaviruses by symbiogenesis of insect DNA viruses in endoparasitic wasps - PubMed for an example, in which the virus-like particles that some wasps use as a component of their venom are described. Similarly to how, even in the article I linked and in this thread, the very important regulatory regions of DNA are called “non-coding” or “selfish genetic elements”, much of the information you hear or read can include bits of old or misunderstood interpretation of the natural processes. I’m sure that whatever I say also includes misinterpretations. As the most successful plasmids will be those that enforce their replication, rather than those that purely prioritize the momentary survival of the cell itself, selection would favor the plasmid which enforces its retention through whatever mechanical means it can develop, as long as they don’t inconvenience the bacterial population as a whole. Thus, what may have started as a genetic element has developed a form of self-interest that runs opposed to that of the cell itself, and the demands of survival have transformed a genetic element to something closer to a symbiote. We must remember that the labels we assign to things are just that, labels, and the things in our world do not actually respect them. Life-like behavior can be observed in parts of life, and competition can happen in environments that we normally don’t consider, such as within a genome or among the organs within a human body. Consider the human placenta, which is an aggressive monster. Genderwise genetic imprinting, a mammalian innovation, allows the male and female parent to compete within the environment of the genome, for their personal reproductive interests. Genomic imprinting - Wikipedia There’s the wiki page, if you’d like to read it.
This post is probably too rambling and confusing to really get any enlightenment out of, but my point here is that the pressures of selection apply equally to all things, and the only things that continue to exist are those things that outcompete the things which might occupy the same niche in the universe, regardless of how we might label them as part of some other thing or creature.
I think I’m describing the idea of the holon. I hope this makes sense, and I wasn’t trying to suggest earlier that rocks are alive. I meant that rocks which can not survive, such as star shaped boulders of sodium, do not survive. This is a point I’ve tried to articulate in arguments against intelligent design proponents who describe scenarios such as the opaque lens, where a man suggested that the evolution of the eye in parts would be unlikely because an opaque lens would not work. The simple rebuttal to him would be that because the lens would not work, it would not work, so it would not evolve. Only the lens that works will evolve. I’ll stop. I’ve diverged too far from the topic in an effort to provide broad context for what I’m saying. Thank you for sticking through.
Welcome to the SDMB, Malphustes! Editing is limited to five minutes after the post is first posted. Double posting is not a problem if you’re adding a substantive reply. Stick around, we always need more knowledgeable people.
Plasmids can affect the pathogenicity of the bacteria that host them. For example, E.coli is component of the gut microbiome, however, there are E.coli variants that cause disease. For some of these, the genes that cause the bacteria to become pathogenic are carried on a plasmid.
Similarely, fore some plant-pathogenic bacteria, the pathogenic genes are carried on plasmids.
So while the plasmids do not cause diseases in eukaryotic organisms by themselves, their presence in bacteria renders these bacteria pathogenic.