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The human immunodeficiency virus, commonly called HIV, is a retrovirus
that primarily infects vital components of the human immune
system such as CD4+ T cells, macrophages and dendritic cells. It also directly and indirectly
destroys CD4+ T cells. As CD4+ T cells are required for the proper functioning
of the immune system, when enough CD4+ cells have been destroyed by HIV, the
immune system barely works, leading to AIDS. HIV also directly
attacks certain human organs, such as the kidneys, the heart and the brain leading to
acute renal failure, cardiomyopathy, dementia and encephalopathy. Many of the problems faced by people
infected with HIV results from the failure of the immune system to protect them
from certain opportunistic infections and cancers.
HIV is transmitted through penetrative (anal or vaginal) and oral sex; blood transfusion; the sharing of contaminated
needles in health care settings and through drug injection; and, between mother
and infant, during pregnancy, childbirth and breastfeeding.
AIDS is thought to have originated in sub-Saharan Africa during the twentieth
century and it is now a global epidemic. At the end of 2004, UNAIDS estimated
that nearly 40 million people were currently living with HIV (UNAIDS, 2004). The
World Health Organization estimated that
the AIDS epidemic had claimed more than 3 million people and that 5 million
people had acquired HIV in the same year
Introduction
In 1983, scientists in France led by Luc
Montagnier, first discovered the virus that causes AIDS (Barré-Sinoussi et
al., 1983). They called it lymphadenopathy-associated virus (LAV). A year later,
Robert
Gallo of the United States, confirmed the discovery of the virus, and they
named it human T lymphotropic virus type III (HTLV-III) (Popovic et al., 1984).
In 1986, both names were dropped in favour of the term human immunodeficiency
virus (HIV) (Coffin, 1986).
HIV is a member of the genus lentivirus
(ICTVdb Descriptions, 61.0.6), part of the family of retroviridae (ICTVdb Descriptions, 61). Lentiviruses have
many common morphologies and biological properties. Many species are infected by
lentiviruses, which are characteristically responsible for long duration
illnesses associated with a long period of incubation (Lévy, 1993). Lentiviruses
are transmitted as single-stranded negatively-sensed enveloped RNA viruses.
Upon infection of the target-cell, the viral RNA genome is converted
to double-stranded DNA
by a virally encoded reverse transcriptase which is present in the
virus particle. This viral DNA is then integrated into the cellular DNA for
replication using cellular machinery. Once the virus enters the cell, two
pathways are possible: either the virus becomes latent and the
infected cell continues to function or the virus becomes active, replicates and
a large number of virus particles are liberated which can infect other
cells.
Two species of HIV infect humans: HIV-1 and HIV-2. HIV-1 is the more virulent
and easily transmitted, and is the source of the majority of HIV infections
throughout the world; HIV-2 is largely confined to west Africa (Reeves and Doms,
2002). Both species originated in west and central Africa, jumping from primates
to humans in a process known as zoonosis. HIV-1 has evolved from a simian immunodeficiency virus
(SIVcpz)
found in the chimpanzee subspecies, Pan troglodyte troglodyte
(Gao et al., 1999). HIV-2 crossed species from a different strain of SIV, found
in sooty mangabeys, an old world monkey of Guinea-Bissau (Reeves and Doms, 2002).
References
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