Viruses have millions of years of evolutionary superiority at coming in and taking over a cell. But one of the problems is when a virus comes in, the body recognizes it and eliminates it. The same thing occurs when a common virus is engineered to be a gene therapy vector. This particular vector is able to get into the cells fairly efficiently and take up residence without causing significant immune system problems. The technique has only been tested in mice and in the laboratory.
Actual treatments are still many years away.