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?

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    $\begingroup$ 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. $\endgroup$
    – Phil
    Commented Jul 12, 2018 at 0:52
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    $\begingroup$ @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. $\endgroup$
    – rus9384
    Commented Jul 12, 2018 at 11:37
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    $\begingroup$ 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. $\endgroup$ Commented Jul 12, 2018 at 18:34
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    $\begingroup$ @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. $\endgroup$
    – rus9384
    Commented Jul 12, 2018 at 19:15
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    $\begingroup$ @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. $\endgroup$ Commented Jul 12, 2018 at 22:01

6 Answers 6


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

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    $\begingroup$ 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. $\endgroup$
    – canadianer
    Commented Jul 11, 2018 at 21:16
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    $\begingroup$ 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. $\endgroup$
    – mbrig
    Commented Jul 11, 2018 at 22:42
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    $\begingroup$ @mbrig they're called endogenous retroviruses. They exist in the germ line. Some are beneficial, some are not. $\endgroup$
    – De Novo
    Commented Jul 11, 2018 at 22:56
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    $\begingroup$ @Mbrig I went ahead and added an example to the answer :) $\endgroup$
    – De Novo
    Commented Jul 11, 2018 at 23:03
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    $\begingroup$ 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 $\endgroup$
    – Nicolai
    Commented Jul 12, 2018 at 10:05

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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    $\begingroup$ I'm aware of them, but it's not good for its possessor, as its direct possessor is that bacterium itself. $\endgroup$
    – rus9384
    Commented Jul 11, 2018 at 22:32
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    $\begingroup$ 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. $\endgroup$
    – kadu
    Commented Jul 13, 2018 at 20:08
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    $\begingroup$ 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. $\endgroup$
    – forest
    Commented Feb 20, 2019 at 8:07

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.

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    $\begingroup$ 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. $\endgroup$
    – rus9384
    Commented Jul 11, 2018 at 23:08
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    $\begingroup$ @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. $\endgroup$
    – James T
    Commented Jul 12, 2018 at 7:59
  • $\begingroup$ @JamesTrotter, some viruses are known to suppress intellectual functions. Why could they do the opposite? $\endgroup$
    – rus9384
    Commented Jul 12, 2018 at 11:47
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    $\begingroup$ @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. $\endgroup$
    – James T
    Commented Jul 12, 2018 at 12:20
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    $\begingroup$ @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). $\endgroup$
    – Luaan
    Commented Jul 13, 2018 at 10:01

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


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.


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.


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