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?