How do you tell it when to stop and how fast to replicate? Can it be shut off when it has done enough? Can it replace breast implants? Just thinking of future business.
This is quite complicated. I will take an example of a disease X (metabolic disease) in which a person lacks an important enzyme. Due to this enzyme missing, this patient can have pretty much any problem depending on the enzyme missing and what its job is (not important right now). This enzyme can cause a direct symptom (where this enzyme is needed for a certain process) or an indirect symptom (where the missing enzyme is a part of a greater pathway and if it is missing may cause several other pathways to not perform).
Ok the enzyme is made by genes. In some people these genes are either missing (making no enzyme) or are inactivated (don't make enzyme) or are present but make an enzyme that is not active itself due to being enzyme made but it is not normal because of abnormal structure therefore not functional (Example: if the normal enzyme xxxxb, the abnormal enzymes maybe xxxxf).
Now since the cells of the body are making the enzyme or not making the enzyme in this case, we would introduce a virus in the body with will be genetically engineered and a part of human genome will be inserted into the viruses. This human genome fragment will make the enzyme the person needs that is absent in this person. Now the thing with this is that this type of treatment would most probably be life long since your body will never be able to make the enzyme because we didn't change the body's DNA. How long will treatments last? I don't know. I guess that would depend on the replication speed of the virus and the life expectancy of the virus and also would depend on the types of medication a person may be taking that may also have an affect on the virus. For example, if the person gets sick and takes an antiviral medication, this may kill the virus also, although they would use meds that wouldn't affect these viruses (long way from this now to worry about).
You see, this is not a cure. You will never be able to make that enzyme yourself. We don't play with or change the human genome. Also these people will pass along their genetic material to their offspring as the sex cells are npt changed. The DNA is not changed.
I doubt you would need to shut it off as you asked because a patient will never really be able to start producing the enzyme himself because of missing genes. To stop it, you would simply stop injections of new viruses and allow the one already inside to just die, but then the symptoms will start up again. They will interrogate with the human cells but these cells also have a life expectancy and will die. So viruses will need to integrate with new human cells.
We are still very far from the applications of these techniques. this technology is very promising though.
Here are a couple of interesting links: