Gene therapy is not yet a routine procedure; the technology is tricky and beset with hurdles to overcome. There have only been a handful of successful trials (which are a cause for great optimism), and so the technology is still being tweaked and advanced in laboratories all over the world.
Viruses are the most common vectors as they infect cells easily and dump their genetic load. Scientists hijack viral cells to neutralise their virulence factors and insert the "good" gene that's to be carried into a body. Target cells such as a patient's heart cells or bone marrow cells are infected by the virus vector. It drops its genetic payload containing the therapeutic gene which is taken up by the cell's genome which goes on to produce the correct protein.
There are several different types of vectors available to the scientist wanting to deliver a fully functioning gene into cells to replace a disease-causing mutated gene:
Adenovirus - a double-stranded DNA virus that usually causes respiratory and eye infections.
Adeno-associated virus - single-stranded DNA virus that inserts genetic material into chromosome 19.
Retroviruses - create double-stranded DNA copies of their RNA genomes which can be incorporated in host cell genomes.
Herpes simplex virus - double-stranded DNA virus that infects neurons.
Non-viral vectors include, injecting naked DNA into target tissue and inserting genes into lipid complexes.