Wouldn’t benefiting the host of the virus be better for the virus itself instead of destroying its host?
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>Wouldn’t benefiting the host of the virus be better for the virus itself
No, the only meaningful measure of success for a virus is its ability to propagate itself. That means spreading to as many hosts as possible, which requires making as many copies of itself as possible, and a virus’s means of reproduction—hijacking cellular machinery to make more viruses—is *necessarily* harmful to the host.
> instead of destroying its host
Most viruses we contract don’t destroy us. They temporarily inconvenience us before jumping to another individual.
Certainly it’s in the virus’s interests not to make us *so* sick that we die or isolate ourselves.
But it does still need to make and spread copies of itself, which is going to tend to be damaging. Unless it switches its replication strategy from “spread from host to host” to “remain where you are and just get copied when your host gets copied”.
Which–could happen. It could be argued that the human [IRGM](https://en.wikipedia.org/wiki/IRGM) gene is (partially) descended from a virus that switched strategies. But once that happens, it’s not really a “virus” anymore, just a gene.
A Google search for “beneficial viruses” turned up these top results:
https://hms.harvard.edu/magazine/viral-world/good-viruses-do
https://www.nature.com/articles/nrmicro2491
https://www.annualreviews.org/doi/10.1146/annurev-vi-04-071217-100011
https://www.sciencealert.com/not-all-viruses-are-bad-for-you-here-are-some-that-can-have-a-protective-effect
There may very well be. We don’t know every virus out there, and we certainly have symbiotic relationships with many bacteria.
Actually, a decent chunk of our DNA is made from ancient viral DNA that got stuck in our genomes, and some of that DNA is actually useful. Your saliva, for example, contains an enzyme called amylase that helps you break down starches, and [it turns out that that enzyme is boosted by ancient viral DNA](https://en.wikipedia.org/wiki/Endogenous_retrovirus#Role_in_genomic_evolution) (probably not “intentionally”).
Viruses _are_ helpful. Not on an individual scale, maybe, but viruses are the primary way for genetic information to travel between species, which increases genetic diversity and helps foster evolution over long periods.
It’s also good to remember that bacteria does not affect all species equally. Deadly diseases happen when a virus that isn’t harmful to its usual host ends up infecting a member of a different species. Generally, viruses aren’t designed to kill their hosts, because when the host dies, that virus can’t keep spreading. But when a virus migrates to a new species, it doesn’t know how to keep its new host alive.
So, what ends up happening is that the virus ends up killing as it spreads. Some of the people it spreads to survive, and build up resistance. The virus adapts as it learns to live and spread in its new host species. The surviving hosts reproduce, and a new generation is born with a stronger immune system than the last. The virus, meanwhile, continues to spread until it either dies out, learns to keep its hosts alive, or crosses into yet another species to repeat the process from the beginning.
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