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This question might seem kind of strange, but it's something I've been chewing on for ages, and I can't really think of a good answer.

Viruses just seem to be floating around passively until they encounter a host cell. Then they enter the host cell, wreak havoc and use the host cell machinery to make more copies of themselves. Then these copies float around passively, until they find host cells.

So what's the point of all this? Infecting the host and causing so much trouble for the host, I mean. The only thing I could think of was something like the "selfish gene" hypothesis, i.e. the genetic material of the virus wants to make as many copies of itself as possible, and in order to secure the machinery for replication, it must infect a host cell. But it's not a very satisfying answer, somehow. All other living organisms at least have their own replication machinery. Then what kind of organisms are viruses? (Are they even organisms?) How and why did they evolve and stay around on the earth for so much time (till the present), if they depend on hosts for their replication?

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    $\begingroup$ The simple answer to why anything in biology happens is always: "Because it works". As long as a thing is able to replicate itself, it will do so. Viruses have found many different ways of getting replication to work, and so they continue to replicate until one day it doesn't work anymore (like it should have been the case for poliovirus a long time ago). $\endgroup$ – Armatus May 30 '17 at 8:32
  • $\begingroup$ They float passively until they encounter the right conditions, the same could be said of any spore or seed. The point of viruses is to make more viruses, the point of any biological product is to spread the genes that created it. $\endgroup$ – John May 30 '17 at 20:29
  • $\begingroup$ I don't really think there's a point to anything. Things just happen, and we try to understand how things happen, and the consequences of things happening. Everything you do and think is only but the result of some chemical reaction. Just like how everything a virus does is nothing but the result of some chemical reaction. I do however believe that viruses are a beautiful example of self-preservation. Viral DNA/RNA truly is the most well adapted in terms of finding a way to continue replication. $\endgroup$ – Bob May 31 '17 at 22:39
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Some folks like mimat get a bit pedantic. "Viruses are not life because we've defined life in such a way that viruses are not alive". Circular reasoning much?

The ocean contain something on the order of 10-15 viral particles for every bacteria. Viruses are clearly an integral, if less-understood, part of modern ecosystems. The question of where they came from, and how they relate to the other domains of life (bacteria, archaea, eukarya) is an interesting one to explore, if unsatisfying because we don't have a lot in the way of answers.

Why do humans exist? From our genes' perspective, we exist as replicative machines whose primary function is to make more of our genes, enabling them to exist in perpetuity. It's the same with viruses. They exist because the mechanism of viral replication is effective at making more bits of viral genetic information.

Asking 'why' something arose in the first place isn't a very useful way of phrasing. Mutation in evolution doesn't occur with foresight, instead it's the random element of evolution. (Natural and Sexual Selection being totally non-random.) Asking 'how' is a better question.

Thinking about viruses not being alive because they lack the ability to make everything they need for replication. This is more than a bit pedantic: are then humans not alive because we lack the ability to make some thing we need (vitamins)? Viral capsules are machines built by genetic sequences that allow those sequences to replicate in perpetuity - same as us. I certainly think they're a form of life, and like almost all other life, they are dependent on existing in an ecosystem with other forms of life.

As to where life came from, or a particular type like viruses, very interesting question. LUCA, or the last universal common ancestor, has been inferred to have been a protocell using something like methanogenesis or acetogenesis, had a cell membrane, and either DNA or RNA for storing and replicating genetic information. Going earlier in time to the RNA world gets interesting.

Think about a given genetic sequence. While any given instance is transient and fragile, there may exist billions of copies of it at any given time on earth. An exact copy of that sequence may have existed for hundreds of thousands of years. Inexact copies for millions, and if you are generous and think of gene families, genes may have lifetimes measured in billions of years.

Microbes are far more 'promiscuous' with their sharing of genetic information than we usually think about, as multicellular organisms. You could think of microbes as systems that are very flexible, while multicellular systems have traded that adaptability for in-system optimization. That is, to us viruses make us sick, but to a bacteria a plasmid or virus might be costly, but potentially advantageous if it brings with it new, beneficial genetic information.

Viruses could be thought of as a super-spore. A spore is a dormant, but very tough capsule that allows its parent genetic sequences to survive hostile times, and then resume metabolism and replication when better times or environments come. To a virus, every environment is a hostile one except the insides of their specific target cell. In a way, you could think of a virus as 'sleeping' or dormant until it enters a host cell, and only truly active or alive when it is. Think about the actual genetic information as the 'alive' entity, and anything constructed by it as a machine.

Viruses probably predate LUCA.

Time to get super-super-speculative.

Among many ideas as to how life arose, in particular cell membranes, one hypothesis is that early cell membranes arose abiotically. According to this notion, life only gradually gained the ability to modify and eventually synthesize membranes. The original membranes may not have been composed of modern stuff at all, as long as it performed the essential functions of being a selectively permeable barrier. Early oceanic vents provide a place where protomembrane molecules could have been abiotically generated and condense on porous rocks.

Likewise, there is a question as to what came first, or if this is a good question: metabolism or genetic information replication. How do you have one without the other? Well, again, early oceanic vents seem a good candidate for abiotic metabolism to have begun building precursor molecules. Get a precursor molecule with the ability to make something like it, and voila: replicators! Personally, I think the order is this: replicators first and gradual control of metabolism and cell membranes later. You could almost think of metabolic machinery as simply being the means of recreating the conditions that existed in those early vents, or the porous rocks around them: conditions that enabled replicating machinery to replicate.

Who knows if this concept is true! But if something like this is how life got started, then at some point before luca, the original replicator more closely resembled a virus as it exists inside a cell, than it resembles modern cells. If replicators in their early days lacked much control over metabolism and/or membranes, that kind of sounds like viruses. Before early life 'took off' in its modern cellular state, early genes needed to connect with other genes for replication, but not necessarily for distribution. Maybe this took the form of physical connection via protochromosomes. Perhaps branches of genetic life that became many viruses opted for an alternate strategy.

Multiple genes that are located on the same chromosome (circular or linear, doesn't matter) have opted to link their evolutionary futures as closely as possible. They are literally, physically linked. The link may not be physical: our genes, even if located on different chromosomes, still have linked fates, marred only by recombination and sex. Remember that microbes are much more fluid and genetically flexible. From a gene's perspective, a cell is simply a location with other friendly genes, which which the 1st gene shares an evolutionary trajectory. If the 1st gene happens to be on a plasmid, however, its fate might be more independent, as copies get swapped into other cells. Microbes have many ways of swapping genetic information. To the extent that this information can be swapped around, the evolutionary future of those swapped bits is less dependent on the exact nature of the other, more cellular bits. Viral genes have opted for an even more independent evolutionary trajectory than plasmids and similar structures. I mean, really, what eats a virus? They build themselves protective capsules and are only really vulnerable when they are inside a host cell and actively replicating. If I were a genetic sequence with billions of physical instances at any given moment, and potential information lifespan in the billions of years, that honestly doesn't sound like a bad way to replicate.

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    $\begingroup$ Welcome to Biology.SE and thank you for your comprehensive and expansive answer. Could you maybe add some reference to your claims (peer-reviewed literature is preferred, Wikipedia etc. is very much okay for common knowledge) - this allows others to follow your arguments and increase their understanding by further reading. Please also consider taking a look at what is considered a good answer on this site and take the tour. Thank you! $\endgroup$ – AlexDeLarge May 30 '17 at 9:23
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The are different theories about virus evolution, some speculate that they were evolved before the cells, some speculate they are evolved to spread genetic information, a sort of specialized conjugative pilus others say that they are cellular parasites. Still, none has the final answer yet, but thanks to the boost of DNA sequencing, many new pieces of the puzzle come out lately so keep updating your knowledge!

Whether they are alive or not is also matter of discussion and it pretty much depends on the definition of life itself. If you define life as the property of reproducing, transmitting genetic information and evolve, then viruses are life forms. But if you want to include in the definition that a life form must have all the biomolecular machinery encoded in the DNA (I really disagree on this point), then they are not. To me, they look quite a living thing.

About your sentence:

So what's the point of all this? Infecting the host and causing so much trouble for the host...

I think you are looking only at the "negative" side. Viruses can transfer useful genetic information, plus they are involved with the evolution itself. We humans have lot's of viral DNA in our genome, so no viruses, no humans as we know them. Think about it ;)

There are plenty of resources around to get a better insight please have look at the following links to get started.

https://en.wikipedia.org/wiki/Viral_evolution

https://en.wikipedia.org/wiki/Evolution_of_influenza

http://www.sciencedirect.com/science/article/pii/S0042682215000859

https://link.springer.com/article/10.1007/s00239-013-9601-0

https://www.nature.com/search?order=relevance&q=evolution%20of%20viruses

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  • $\begingroup$ I think you are looking only at the "negative" side. Viruses can transfer useful genetic information, plus they are involved with the evolution itself. Downvoted because of this profoundly misleading sentence which suggests that there's a goal to evolution, and that organisms "should" only evolve if they have "good purposes". $\endgroup$ – iayork May 30 '17 at 11:58
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    $\begingroup$ @iayork Where do you read this exactly? I did not suggest any goal whatsoever. I just suggested that there are positive effects of virus infections, including broader secondary effects like humans evolution. My sentence was meant to point out that you have to consider the variety of effects, not only the "negative" one. But definitely, there is no suggestion of any goal or a "good purpose". $\endgroup$ – alec_djinn May 30 '17 at 12:33
  • $\begingroup$ My sentence was meant to point out that you have to consider the variety of effects, not only the "negative" one. Exactly my point; that's a profound misunderstanding of evolution. $\endgroup$ – iayork May 30 '17 at 12:36
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    $\begingroup$ @iayork how can be misleading something like "consider the variety of effects". I really don't get you... $\endgroup$ – alec_djinn May 30 '17 at 12:43
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    $\begingroup$ That's definitely not a mature nor scientific way of putting it. $\endgroup$ – alec_djinn May 30 '17 at 12:57
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Viruses are not considered alive because they cannot reproduce independently and are therefore not organisms.

As to the answer to 'why' something arose in evolution is always the same: because at that moment in time this specific set of traits was able to function and create a lot of offspring (or copies). The virus 'might create a lot of trouble for the host' but that does not matter, a lion causes a lot of trouble for antelopes but that does not get questioned.

Evolution as a process is not teleological, nothing evolution does has a purpose or end goal, a combination of traits simply works better than what came before in a specific set of circumstances.

You already mentioned the selfish gene hypothesis, in my personal opinion that is the best explanation.

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  1. Viruses actually have RNA. Due to this property of genetic material, RNA organisms are faster than DNA organisms!

  2. Secondly, they have a protein coat that acts as another important thing which makes allows them to live in contact with organisms!

  3. But every coin has two sides. It is much used in biotechnology to transfer important genes in plants and animals by disarming their pathogenic properties!

  4. When they come in contact with host DNA, host DNA starts to replicate its virus genetic material and then its own! For example, in the case of HIV.

enter image description here

See in the image how reverse transcription the most important process converts RNA -> DNA then creates an infected DNA strand in host and host produces its RNA which is infected!

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  • $\begingroup$ Hi Welcome to Bio.SE! Please provide some support for you answer to avoid having it downvoted or closed. Supporting answers with credible references is a best-practice that we have come to expect on this Stack Exchange site. Thanks! $\endgroup$ – theforestecologist May 30 '17 at 21:14
  • $\begingroup$ Your answer doesn't seem to answer the question. Please edit your answer to avoid having it downvoted/removed. See here for a guide to writing good answers. ... Also, I tried editing a bunch of your answer to try to improve grammar, spelling, punctuation, etc. I hope that you use this as a learning tool to create higher quality posts of your own in the future. Thanks. $\endgroup$ – theforestecologist May 30 '17 at 21:22
  • $\begingroup$ There are a lot of factual errors in this answer, you have both RNA and DNA viruses and these have little to do with speed. Naked viruses do not have any proteins so that is also wrong $\endgroup$ – mimat May 31 '17 at 10:02

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