Thinking about the virus that causes AIDS, which at least until recently was a sure death sentence, I wondered what would it benefit a virus to kill off its host. Wouldn’t it be evolutionarily superior for a virus to instead just hang around until the end of the host’s natural lifespan?
But if that’s so, why don’t we ever hear of any good viruses? If a virus had any positive impact on people, it wouldn’t be cured or avoided. Yet we have only bad ones?
Because a virus doesn’t need to be good, only good enough to pass on its genetics and create a lot more viral particles. Evolution selects for whatever is most effective at making copies of itself that are, themselves, most effective at making copies of themselves.
That said, there are viruses that will normally never kill you but are effectively impossible to root out entirely. Herpes is a prime example: One case of chickenpox and you are subject to cold sores and/or shingles the rest of your life, because the virus that causes all of them never really leaves you but is effectively dormant most of the time.
One class of viruses, called bacteriophages, only attack bacteria. Each strain of phage only attacks a specific species of bacteriom. As such, we can use these to combat various bacterial infections; I’d call this a "good"virus if that term has any meaning.
related thread from a couple years ago:
Actually, about 1-2% (some sources say up to 8%) of the human genome is of viral origin, the result of ancient infections by retroviruses (the same class of virus as HIV). Retroviruses integrate their genes into the DNA of the host. When they infect the reproductive cells, they can be passed down as endogenous retroviruses (ERVs).
It is thought that such a viral infection may have aided the evolution of viviparity in mammals. From here:
Some scientists have also proposed that retroviruses also played a role in the divergence of humans ancestors from chimpanzees.