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.