What is HIV?

In 1981 an epidemic of a previously unknown acquired immune deficiency syndrome (AIDS) was described in the USA. A lentivirus (subfamily of retroviruses) was subsequently identified. Lentus (in Latin = slow) denotes the long latent phase between infection and the development of symptoms. Retroviruses use the enzyme reverse transcriptase (RT) to generate proviral DNA from RNA (reverse of the usual direction of genetic transcription). The term human immunodeficiency virus (HIV) was accepted in 1986. In the same year a related virus (HIV-2), endemic in W. Africa, and sharing common features including the induction of immune deficiency, was identified.

Origin of HIV

There are structural and genomic organizational similarities between HIV and simian immune deficiency virus (SIV). Despite similarity between HIV-1 and 2, there is little sequence homology with HIV-2 more closely related to SIV than to HIV-1. Phylogenetically, HIV-1 and HIV-2 cluster with chimpanzee (Pan Troglodytes) and sooty mangabey simian retroviruses, respectively. SIVcpz is almost identical to HIV-1. It appears likely that the virus at some point crossed species from chimpanzees to man.

Evidence that the virus existed for some time before its effects became clinically apparent in 1981 is supported by the following:

  • HIV detected in a blood sample taken in 1959 from an adult male living in the former Zaire.
  • HIV found in tissue samples from an African American teenager who died in St Louis, USA in 1969.
  • HIV found in tissue samples from a Norwegian sailor who died around 1976.

Prevalence of HIV

38 million people, 2.1 million of them children, were living with HIV/AIDS by the end of 2003 (two-thirds in sub-Saharan Africa). An estimated 4.8 million new infections occurred in 2003.

The number of people living with HIV is ↑ because of the continuing epidemic and the availability of life prolonging drugs. Infection rates are likely to ↑ in countries with poverty, inadequate healthcare, and limited resources for prevention. The socio-economic impact is greater in developing countries.

HIV Viruses and Their Epidemiology

HIV-1 and HIV-2

Differ in several aspects:
  • HIV-1: a rapidly mutating virus eventually producing divergent quasi-species. More virulent and rapidly progressive than HIV-2.
  • HIV-2: predominantly found in W. Africa but has been recently reported in India and S. America. ↓ viral loads (VLs) independent of the duration of infection. Rate of vertical transmission ↓ than HIV-1.
HIV-1 and 2 are classified into groups according to their genetic diversity.


  • Group M (main group): further divided into subtypes or clades with at least 11 genetically distinct subtypes, A1, A2, B, C, D, (E now considered a circulating recombinant form, CRF01_AE), F1, F2, G, H, J, and K (I now considered a circulating recombinant form). Some variants are termed U category i.e. uncertain or unclassifiable, which may represent new subtypes, or recombinant forms. At least 15 circulating recombinant forms have been described so far and are likely to increase as is their proportion in the pandemic.
  • Group O (outlier group).
  • Group N (new group).


Divided into group (rather than subtype) A to G.

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