How does gene therapy work?

I’ve heard of therapies where somehow genes are manipulated by one method or another so that faulty genes are repaired and the patient is cured.

How does this work? After all, you can’t manipulate all the cells in the body or even in one organ, so you can only do some limited set. Then you have to hope that somehow the treated cells replace the untreated ones, but it seems to me it could just as easily go the other way.

Can anyone explain how this is supposed to work?

I am not a doctor, but one way would be to construct or modify a virus that injects the proper DNA sequence into the host cell without destroying it. Breed the viruses in some sort of a special solution and “infect” the patient, modifying the DNA in all the cells the viruses get to. This is just a WAG though, I don’t know if (a big if) how it’s actually done.

It depends on the disease. Say, for instance, that your disease is caused by the lack of a certain protein floating around your blood, because your gene for that protein is faulty. But if you can get even a small percentage of your cells to start expressing the correct gene for that protein, they could make enough of it to cure your disease or at least alleviate the symptoms.

You’re right that it’s pretty much impossible to fix all of your cells. Gene therapy is going to be most useful in situations where you don’t need to fix them all.