We're actually on the cusp of incredibly cheap genetic modification, using a technology called CRISPR. The short version, is we can modify bacteria, with regard how it fights viruses, to rip out unwanted slices of DNA, while, at the same time, introducing corrected pieces to fill in the gap.
How cheap do I mean by cheap? It costs about $25 in the lab to do in small scale (cellular treatment in a petri dish, not enough to work on an entire adult, and not sequenced to a specific individual's DNA, total, we might be looking at a couple thousand dollars plus profit for those involved).
Scientists are still working on it, but some are already working on genetically engineering other species with it and are talking about using it on human embryos too... imagine a world without AIDS, cancer, and other disorders that have a genetic markers - either through their own virus mechanisms or human chromosomes.
We could have a world without asthma, cystic fibrosis, hemophilia, sickle-cell, etc... even something as trivial as baldness can be eliminated from the gene pool. Sounds great, right?
We could also theoretically have a world without gay people, transgender people, etc (ok, not all of that is necessarily genetic-only, there's also evidence to show for in vitro processes causing it)... to borrow a word from Gattaca, no more "de-generates."
Welcome to the world of eugenics, where everyone is either identically perfect or an outcast. That sets up another discussion about monoculture and long term species survivability and whether it's healthy to put all of our eggs in one basket. As the plague spread across Europe, humanity survived, in part, because some people had a genetic difference that made them more resistant to it.
In the end, technology is just a tool and it has no will of its own. It can be used for good or evil... it's up for us to decide what is good or evil, and whether the societal cost is worth it.