Viruses aren't strategic thinkers. If one virus is too deadly to survive, another variant will take over.

The virus responsible for the 1918 influenza pandemic still circulates today, but in a far less deadly form. Viruses reproduce so quickly and so often that they are self-correcting and rarely disappear altogether. Those that are too deadly disappear and others survive.

Because they’re like wasps, just exist to mess shit up.

But really I think it’s mostly because they aren’t meant to kill their “intended” host. Like viruses that exist in a specific species won’t kill that species, but when it crosses to a different one that organism can’t fend it off as well and ends up succumbing to it.

The short answer is: because it doesn’t need that host to survive.

Viruses are trying to replicate as quickly as possible using host cells. A virus that spreads easily and quickly can kill its hosts as long as it has had a chance to reproduce and infect another organism first.

The ones that don’t succeed in doing this die out.

Because the vast majority of viruses are able to infect an additional host before killing the first one (if it even kills at all), killing the host is not a genetic disadvantage.

dont take viruses are strategic thinkers, their only goal is to hijack the host's cells in order to replicate and the fate of their host is not something that's conisdered unless it hinders its aiblity ot do the 1st.

evne if the host dies as a result of the virus, if its not disrupting itsd aiblity to hijack its aiblity ot replicate there is no pressure to change.

A lot of them don't nessessarily kill us. Our own immune system trying to fight the virus does itself in.

what host 'needs' to survive? The host just needs to live long enough to pass the virus along.

Viruses aren't thinkers, they are pushed by laws of natural selection, and those laws can find more than one "good enough" solution, and they often do.

It's true that natural selection often pushed viruses to be more mild but the exact opposite can happen, where natural selection pushed viruses to be super heavily virulent, infect millions of cells, make billions of viruses, cause the organism to spread the virus to a lot of other organisms, and finally die off. Thing is, at least in humans, these viruses "burn" through population rather quickly, and then population becomes immune to them.

Check out deadly yet not-so-dradly viruses, like measals or smallpox. If population of humans has never been exposed to these diseases previously, they will absolutely ravage that population, think native Americans after European arrived. That's maybe 90% of population dead. Yet for most of us measals is not that deadly.

Viruses aren't really alive in the way that we think of it, and don't have any particular drive to do anything. The interaction of a host and a virus results in more virus being made, so that virus continues to exist, but it's neither trying to kill its host or to survive.

I don't think anyone has mentioned it yet, but almost all the viruses that actually kill the host are viruses from other animals that spilled over into the human population.

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