Gene therapy is a pretty promising approach for lots of different diseases, and has already overcome a huge hurdle with the approval of the very first gene therapy product, Glybera, by the European Commission in 2012.
At its core, gene therapy uses a delivery vehicle to deposit a chunk of DNA in particular cells of the body. That courier is often a virus that has evolved to penetrate cells incredibly efficiently, and its genetic payload can be designed to normalise a cell if it's simply defective, or to kill it if it's cancerous.
But taking a human virus and reprogramming it for good rather than evil is not a trivial task. You have to be sure that there's no risk of inadvertent infection and nasty disease, and you need it to stick around long enough to do its job. That means it has to be able to avoid detection - and destruction - by the immune system.
Happily, German scientists discovered in 1995 that an insect virus, baculovirus (shown below), is able to enter, but not replicate in, human cells. So there is no possibility of unintended illness, and since humans aren't typically exposed to insect viruses, they don't come under immediate memory immune attack.