Thus far, four human retroviruses — HTLV 1 and 2, in conjunction with HIV 1 and 2 have been found to attack Helper T

Studies of retroviruses led to the first demonstrated synthesis of DNA from RNA templates, a fundamental mode for
transferring genetic material that occurs in both eukaryotes and prokaryotes. It has been speculated that the RNA to
DNA transcription processes used by retroviruses may have first caused DNA to be used as genetic material. In this
model, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create DNA from the
RNA templates.

The following genera are included here:
Genus Alpharetrovirus; type species: Avian leucosis virus
Genus Betaretrovirus; type species: Mouse mammary tumour virus
Genus Gammaretrovirus; type species: Murine leukemia virus; others include Feline leukemia virus
Genus Deltaretrovirus; type species: Bovine leukemia virus; others include the cancer-causing Human T-lymphotropic
Genus Epsilonretrovirus; type species: Walleye dermal sarcoma virus
Genus Lentivirus; type species: Human immunodeficiency virus 1; others include Simian, Feline immunodeficiency
Genus Spumavirus; type species: Chimpanzee foamy virus

The virus itself stores its nucleic acid genome and serves as a means of delivery of that genome into cells it targets as
an obligate parasite, and constitute the infection. Once in the host's cell, the RNA strands undergo reverse
transcription in the cytosol and are integrated into the host's genome, at which point the retroviral DNA is referred to as
a provirus.