We know that Virus particles are small chemical objects between living and non-living things and that the are the infective agent in some illnesses.
However a virus works by injecting a cell with its genetic material then inserting its code into the nuclear DNA and then making thousands of copies of itself using the cells machinery.
We know that genetic diseases are caused by defects in the genetic material of a carrier or sick person.
It will be possible for us to use the infectivity of an artificial virus to infect a diseased cell and correct the defective genetic material.
We need to design a virus to be able to infect one cell and alter the appropriate gene code then not replicate itself except during normal cell division so that the disease is cured. It would need to be able to find adjacent DNA code and then snip out the damaged or missing sequence and insert the corrected code. Naturally occuring viruses are able to find a region, insert themselves and then stay there for several cell cycles (lysogony). An artificial theraputic virus would need to be made by modification of this ability.
Treatment would be an injection of virus particles (so designed as not to cause an immune response) in sufficient quantity to that every cell is infected once and the genetic transformation takes place to cure the disease.
Illnesses that might quickly succumb might be huntingtons cholera, retinosea and that disease that causes the lungs to fill with fluid caused by faulty chloride metabolism.
Later genetic disorders causing deformity might yeild and illnesses like insanity and downs syndrome.
Eventually even old age will become curable.