(Also retro-virus) A virus with which the usual direction of flow of information is reversed, being transcribed from its RNA into DNA in the host cell. HIV is the most interesting of the natural retroviruses.
DNA is formed from RNA using a reverse transcriptase, which is an essential part of the retrovirus and not so far found in human cells. Integrase is needed for the DNA copy to add itself into the cell's DNA.
The selective advantage to a retrovirus in this complex arrangment presumably lies in the possibility of lying quiescent in the cell's genome and being activated after some considerable time, perhaps when conditions are favourable to viral propagation.
Genetic engineering techniques using engineered retroviruses to patch the genome of somatic cells are conceivable but considerable technical obstacles exist as well as the worry that inserting segments of code may have unforeseen consequences on the rest of the cell's functions, including liberating other viruses or triggering cancer.
Rapid mutation and evolution
RNA viruses mutate more rapidly than DNA ones. Retroviruses tend to mutate even faster, amplifying the problem of drug resistance in HIV. This also has social elements.