Retroviruses have a wide distribution in nature, with examples in insects, reptiles and nearly all mammals. The human retrovirus, human immunodeficiency virus (HIV 1 and 2), belongs to the lentivirus group of the retrovirus family, whilst human T-cell lymphotropic virus (HTLV I and II) belongs to the oncorna group. Human T-cell lymphotropic virus I and II are thought to have evolved from simian T-lymphotropic retroviruses that were transmitted to humans over the past centuries or millenia. Human immunodeficiency virus is thought to have derived from simian immunodeficiency viruses that are endemic in chimpanzees in Central Africa, and probably infected natives over the past century (Sharp et al., 2001).
Retroviruses are membrane-coated, single stranded RNA viruses that have a distinct genomic organization and require the presence of reverse transcriptase in their replication cycle. In a typical infection, retrovirus particles attach to the cell membrane, reverse transcriptase copies viral RNA into complementary double stranded DNA and this is integrated into the host cell chromosome. Host cell enzymes help virus and host regulatory genes complete the retrovirus lifecycle by producing virions that bud from the plasma membrane to infect other cells or organisms.
Human immunodeficiency viruses 1 and 2
Definition and characteristics of agent
Human immunodeficiency virus was discovered in the early 1980s by two groups of workers, Montagnier in France and Gallo in the USA. Originally described as human T cell lymphotropic virus type III (HTLV-III), the virus was shown to infect T-cell lymphocytes.