Using viruses to treat altered or misconfigured DNA Consider how a Retrovirus can modify existing cell DNA to 'execute instructions' on its behalf. I wondered: Why can we not utilize lab-generated viruses to infect sick patients with a 'healthy' virus that would rewrite bad segments of DNA with something either correct or non-malus? This of course is assuming that we have a solid understanding of the relationship between genomes and precise viral functions, additionally that we had some sort of gene replacement functionality.

> Consider how a Retrovirus can modify existing cell DNA to 'execute instructions' on its behalf.

Not only retroviruses do that. Actually, pretty much all viruses use the machinery of the host cell on its behalf.

> Why can we not utilize lab-generated viruses to infect sick patients with a 'healthy' virus that would rewrite bad segments of DNA with something either correct or non-malus?

We actually do that. We use viruses (incl. retrovirus but not only) to insert DNA segments into eukaryote genome. See vectors in gene therapy > viruses

> Retrovirus to reverse another retrovirus

Note that I don't really understand your title so, I ignored it a bit focusing on the content of the post.