Retroviruses are enveloped, positive  . The virion enclosing the capsid contains 2 copies of the RNA genome. Replication proceeds through a DNA intermediate (provirus). There are 3 following subfamilies of human retroviruses.

Oncoviruses: include:

human T cell lymphotropic (leukemia) viruses I (HTLV-I): was discovered some 6-7 years before HIV and is associated with adult T cell leukemia and HTLV-1 associated myelopathy. It is endemic to southwestern Japan, Carribean, Africa. HTLV-1 has a C like nuclear capsid. Structurally it is similar to HIV. Its envelope is smaller and has the proteins Gp46 instead of  and Gp21 instead of . Its regulatory genes are Rex and Tax instead of . Methods of transmission are the same as with HIV (with transmission during breast feeding thought to be important for pathogenesis). 

HIV-1 may be a co-factor in accelerating progression to AIDS. In addition, other inflammatory disorders are potentially associated with HTLV-1 infection including Sjogrens syndrome (automimmune destruction of the salivary glands), arthritis, dermatitis and non-Hodgkins lymphoma.

HTLV-II: shares almost 50% overall homology with HTLV-1. It has no strong association with human disease and a high incidence of infection among IV users. 

Lentiviruses: infect many different mammalian species. Infection with lentiviruses go through 3 stages; (1) an initial (acute) stage associated with rapid viral replication which is often accompanied by a transient period of disease, (2) a latent period during which the virus is brought under some immune control and no disease occurs and (3) high levels of viral replication later followed by disease. 

All lentiviruses also persist for life and have high mutation rates. Lentiviruses replicate in the nucleus. Virus assembly is in the cytoplasm.

The 3 major groups of primate lentiviruses are: 

HIV II

Simian immunodeficiency virus (SIV): SIV is a model for human HIV infection. 

Spumaviruses: include the foamy viruses.