Question

Describe the process of altering the genetic variation of a cell using a genetically modified virus for human gene therapy. In the future, how may this effect the gene pool?

Answer 1

Basically, it is really hard to stick new genetic material into human (as well as other eukaryotic) cells without killing them. Which defeats the purpose - new genetic material in a dead cell is useless. It is a lot easier to get new genetic material into a bacterial cell or into a virus (viruses are really easy, you can't kill them because they are not even technically alive).

And because bacterial cells and viruses infect human cells - that is, they get their genetic material into human cells without immediately killing the human cells - they are a far more effective mechanism for getting target genes and sequences into human cells without killing them.

There are far more details than that (people do doctoral dissertations on this) - for further reference read up on "gene knock in" and "gene knock out" strategies...