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Typically, people call viruses some kind of organic compounds that cannot reproduce autonomously and which lower the fitness of their hosts. Even the word "virus" means "venom" in Latin.

But from the perspective of natural selection, one would expect those organic compounds that cannot reproduce autonomously, but which would increase the fitness of their hosts, to be more widespread. One can see an analogy with bacteria: people are more aware of harmful bacteria and even such words as "microbe" are perceived as somewhat harmful (among non-biologists for sure). But we know that an animal body contains many more useful bacteria than harmful ones, and animals have their own microflora, which are necessary for survival.

The same must be true for viruses: those viruses which were useful (or at least unharmful) to their hosts would be passed more easily to other organisms since their hosts would have a selective advantage.

So, do such beneficial for their direct hosts viruses exist? If so, what are they called? What are the examples?

rus9384
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    Technically, since we do not know if a virus is alive or not, we cannot say it is "good"; akin to not being able to say a rock has good behavior. – Phil Jul 12 '18 at 00:52
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    @Phil, the question is not about the morality (and I even do not make such a distinction), and this is not a Phi.SE. Under "good" I mean its positive value for its possessor. Just like the water is good for you when you are thirsty. – rus9384 Jul 12 '18 at 11:37
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    Someone's been watching Red Dwarf :P – Ruadhan2300 Jul 12 '18 at 15:19
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    Are you only looking for naturally occurring viruses in your question? We've engineered a few viruses (from existing viruses) which can overcome certain diseases. – Reginald Blue Jul 12 '18 at 18:34
  • @ReginaldBlue, that's also is interesting topic, would be interesting to read about that. – rus9384 Jul 12 '18 at 18:36
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    @russ9384: Well, then do not use the word "good" and keep the focus on fitness as you mentioned or other word like "survival probability" and "probability of passing on its genes"- terms that are objective and focus on things in terms of biology. One would think you are making this a "Phi.SE" discussion, when you throw in "'Phi.SE' terms" into a molecular biology discussion. This is a minor point that contributes littles to your goal. I think we can now end this. – Phil Jul 12 '18 at 19:11
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    @Phil, I couldn't predict that one would think of the word "good" as only in moral terms. I thought it's common to use the word "good" interchangeable with "useful". Because it's really common in my place to do it. But "useful" would be misleading as then bacteriophages would be useful. – rus9384 Jul 12 '18 at 19:15
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    @rus9384 - The word "good" has a variety of meanings. In general, readers will know that many words have multiple meanings. Choosing the intended meaning is often easy, as in this case. Nevertheless, you could substitute the word "beneficial" if you're looking for something more precise. – Don Branson Jul 12 '18 at 22:01
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    One of the theories of how viruses came to be in the first place is that they started as symbiotic organisms - perhaps in a similar way that mitochondria or chloroplasts are thought to be. Over time, they lost most of their machinery until possibly only the barest genetic coding remained, and either simply turned parasitic, or left the cell's environment and moved into other cells that weren't quite as welcoming (note how most human plagues are caused by viruses that were transmitted from other animals - the same virus that caused mild symptoms in e.g. a cow would kill a human). – Luaan Jul 13 '18 at 10:10
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    I have gone ahead and made the beneficial edit discussed. We really cannot have questions imputing moral qualities to viruses when a functional descriptive adjective will do. Sloppy writing encourages sloppy thinking. – David Jul 15 '18 at 18:06
  • Even reworded I feel there's still a certain amount of ambiguity to this question. Contracting Chicken Pox is "beneficial" if you consider the benefit of the immunity you gain. Regular contact with Influenza is said to benefit the overall strength of the immune system as well, though tbh, I'm not sure if that's a myth or not. I see these types of benefits as analogous to the way lifting weights creates a short detriment but a long term benefit. I'm not posting this as an answer because I know practically nothing about the subject - more just thinking out loud and wondering here... – Eaten by a Grue Jul 16 '18 at 15:24
  • @billynoah, I believe the word "good" itself created no much more ambiguity. In either way adding "for their direct hosts" after "beneficial" made it less ambiguous, right? – rus9384 Jul 16 '18 at 15:50
  • yes, it's less ambiguous. do my examples above qualify as "beneficial" according to your intent? – Eaten by a Grue Jul 16 '18 at 15:56
  • @billynoah, regular pain reduces the sensitivity to pain. I can't say it's really beneficial. The mechanism is beneficial, but that's mechanism, not the disease (in your example) itself. – rus9384 Jul 16 '18 at 16:07

6 Answers6

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Do they exist? Yes

What are they called? Marilyn Roossinck calls them viral mutualistic symbiotes. She has an excellent review here.

What are some examples?

My personal favorite is GB-Virus C, or Hepatitis G, which appears to slow the progression of HIV using a number of different mechanisms:

Box 1. Summary of the effects of GBV-C infection in HIV-positive individuals

  • GBV-C infection downregulates HIV entry co-receptors CCR5 and CXCR4, and increases secretion of their ligands RANTES, MIP-1α, MIP-1β and SDF-1.
  • In vitro GBV-C NS5A and E2 proteins inhibit X4- and R5-tropic HIV replication, and NS5A protein downregulates CD4 and CXCR4 gene expression.
  • HIV-infected individuals positive for GBV-C E2 antibodies have survival benefit over HIV-infected individuals with neither GBV-C viremia nor E2 antibodies; in vitro GBV-C E2 antibodies immunoprecipitate HIV particles and inhibit X4- and R5-tropic HIV replication.
  • GBV-C induces activation of interferon-related genes and pDCs.
  • GBV-C promotes Th1 polarization and the NS5A protein contributes to this effect.
  • GBV-C infection reduces surface expression of activation markers on T lymphocytes, suggesting its role in T cell activation signaling pathways.
  • GBV-C protects the T cell from Fas-mediated apoptosis and as a result of its effect on immune activation may also play a role in protecting lymphocytes from activation-induced cell death.
  • GBV-C viremia reduces IL-2-mediated T cell proliferation suggesting a significant interaction between GBV-C, IL-2 and IL-2 signaling pathways.

Endogenous retroviruses

As @mbrig recalls in the comments, there are a number of retroviruses that have inserted themselves into the germ line. Those are called endogenous retroviruses, and they interact with the host genome in a number of ways. Some are even translated:

Proteins produced from ERV env genes have also been demonstrated to function as restriction factors against exogenous retroviral infection

De Novo
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    Interesting answer. For future reference, it is not ideal to post text as images: it increase page loading time, prevents searching and presents various accessibility problems. – canadianer Jul 11 '18 at 21:16
  • Oh, wow. Now it is interesting to know whether my hypothesis that these viral mutualistic symbiotes are integral part of almost any life form, like how micrpflora is such for animals. And hypothesis there are much more viral mutualistic symbiotes than viruses (which are causing diseases). But this is not within the scope of this question of course and has not been yet investigated, I believe. – rus9384 Jul 11 '18 at 21:17
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    @canadianer ok :) i typically post images of figures, and pulled a box instead. You're right this is better as quoted text. Thank you for making the edit. – De Novo Jul 11 '18 at 21:18
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    Though I can't name any off my head, I have fuzzy memories of reading that a number of retroviruses have become permanent (and beneficial/essential?) parts of some species genomes. – mbrig Jul 11 '18 at 22:42
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    @mbrig they're called endogenous retroviruses. They exist in the germ line. Some are beneficial, some are not. – De Novo Jul 11 '18 at 22:56
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    @Mbrig I went ahead and added an example to the answer :) – De Novo Jul 11 '18 at 23:03
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    These retroviruses (or by now also retrotransposons) are so ancient, that by now we have a couple of 'human' (and fly and ...) genes that probably evolved from them. One crazy example is Arc, which is important for brain developement and seems to have a very uncommon mechanism – Nicolai Jul 12 '18 at 10:05
  • @Nicolai I think that might have even been the one I read about, thanks! Amazing that a virus ends up not just harmlessly incorporated, but actually useful/needed :) – mbrig Jul 12 '18 at 15:42
  • Endogenius retroviruses part apply perfectly for analogy with microflora. – rus9384 Jul 12 '18 at 17:30
  • @rus9384 well, they are germ line viruses (I"m not sure there is consensus on the distinction between an endogenous retrovirus and a retrotransposon), so I don't know if I'd agree that they are perfectly analogous to microflora. The gut virome and skin virome might, but much of that is comprised by phage viruses, so that's not perfect either. – De Novo Jul 12 '18 at 17:37
  • Transposons cannot be passed to other individuals (except offsprings), right? I thought a virus is a virus, because it can be passed. – rus9384 Jul 12 '18 at 17:42
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Another good virus would be a Bacteriophage, a virus that infects and kills illness-causing bacteria. From Wiki:

A bacteriophage also known informally as a phage, is a virus that infects and replicates within Bacteria and Archaea. The term was derived from "bacteria" and the Greek φαγεῖν (phagein), "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures.

They have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe as well as in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria.

Intentionally using Bacteriophages medicinally is called phage therapy.

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Astor Florida
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    I'm aware of them, but it's not good for its possessor, as its direct possessor is that bacterium itself. – rus9384 Jul 11 '18 at 22:32
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    A bacteriophage is a virus that infects (not "eats") bacteria. That also doesn't mean they're good for the bacterial host. For instance, V. cholera does not produce the toxin that causes the symptoms of cholera unless it has been infected by the CTXφ bacteriophage. Along with its viral DNA, it also inserts the cholera toxin gene, turning the bacterial host (V. cholera) deadly for its own host (the unfortunate person who contracted the disease). This could arguably be considered "good" for the virus's host, since the definition of "good" seems pretty arbitrary in this context. – kadu Jul 13 '18 at 20:08
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    Phage therapy is largely ineffective given that not only can it increase the pathogenicity of bacteria (as @kadu points out) and can aid in spreading resistance factors (R plasmids), but because it triggers an extremely strong immune response against the otherwise harmless phage virions. – forest Feb 20 '19 at 08:07
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I would say that if any "good" viruses exist, they are already within us. Retrotransposons are genetic elements in our DNA that were likely ancient viruses and they move around from time to time either by excising themselves and moving somewhere else or by making a copy and inserting it somewhere else in the genome. Even though we are born with them, their activity is similar to modern viruses. When retrotransposons insert themselves in a new place they can cause disease, but the variation they cause likely brought us some beneficial advantage at some point because we have so many of them and they stuck around this long.

user40950
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    Interesting point. I'm just thinking about how plausible would it be to get increased intelligence, for example, from having sex with someone (sounds hilarious, I know), such kind of sexually transmitted advantage. – rus9384 Jul 11 '18 at 23:08
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    @rus9384 I don't believe the effects would be immediate, I'm not sure any kind of gene therapy can "increase the intelligence" of an already fully developed brain. – James T Jul 12 '18 at 07:59
  • @JamesTrotter, some viruses are known to suppress intellectual functions. Why could they do the opposite? – rus9384 Jul 12 '18 at 11:47
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    @rus9384 because it's easier to switch a light off than it is to manufacture and install a circuit, bulb & switch and then switch it on. – James T Jul 12 '18 at 12:20
  • @JamesTrotter, better example then could be metabolism... Any person with slow metabolism in developed society (who is either overweight or need to be on diet or go to gym regularly just not to be overweight) would like to get a virus of accelerated metabolism. – rus9384 Jul 12 '18 at 12:24
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    @rus9384 Well, now you run into another complex problem - there's no such thing as "good" or "bad" without context. The same virus that would "cure" your obesity would kill you if you were poor (not to mention that a lot of viruses and bacteria already do that - a serious illness can make you lose quite a bit of weight). – Luaan Jul 13 '18 at 10:01
  • @Luaan, would those people who are skinny even if they eat much, die if they would eat less? I'm pretty sure the mechanisms of those illnesses are different. – rus9384 Jul 13 '18 at 10:04
  • @rus9384 There's probably lots of variants. Which comes back to the original intent - context changes everything. I'm pretty sure it's incredibly unlikely for a natural virus to be 100% beneficial - remember, all they can do is make your cells produce proteins. Maybe they could make a diabetic's cells produce insulin in just the right amount - but at the same time being harmless to a non-diabetic? That seems to be stretching the probabilities way too much (unless you look at a population after everyone else was killed off :P). Of course, we might engineer such viruses, possibly. – Luaan Jul 13 '18 at 10:20
  • @Luaan "all they can do is make your cells produce proteins" ... those proteins can do an awful lot, and transcription, translation, processing, distribution, activity, and turnover are all subject to co-evolving regulation. – De Novo Jul 16 '18 at 15:38
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We have engineered a few good viruses to treat certain diseases

Per my comment and response:

The most current example (at this time and based on my recollection) is the virus we have engineered to treat a certain type of macro degenerative eye condition:

Scientists Have Reversed Age-Related Blindness by Deliberately Infecting Eyes With a Virus

There are a few other cases out there, and, as always, xkcd is there to help:

T Cells

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Reginald Blue
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Cowpox and smallpox viruses are structurally similar, and catching one confers immunity to both by immune system response, but one was a deadly disease and the other almost harmless. Once this was discovered, the days of smallpox were numbered. We had the means and the motivation to stamp it out.

On my last check a few years ago, we are deliberately keeping cowpox alive to ensure that we can kill smallpox should it ever come back.

The virus that ended a plague is a good virus.

bandybabboon
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Joshua
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So, do such beneficial for their direct hosts viruses exist? If so, what are they called? What are the examples?

Bacterial viruses, termed bacteriophages or simply phages are normally killers of bacteria and thus occasionally useful for humans (e.g. phage therapy) but not beneficial for their hosts. However, some phages, called temperate phages, have a lysogenic life cycle, where the viral genome can become integrated into the bacterial chromosome and they replicate with the bacteria (at least temporarily) as a prophage. These prophages have a shared evolutionary fate with the bacteria and can be directly beneficial by carrying bacterial fitness factors such as virulence factors or antibiotic resistance genes. You might be interested to read more in our short review.

Chris_Rands
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