HIV is a highly variable virus which mutates very readily. This means there are many different strains of HIV, even within the body of a single infected person.

Based on genetic similarities, the numerous virus strains may be classified into types, groups and subtypes.

Difference between HIV-1 and HIV-2
There are two types of HIV: HIV-1 and HIV-2. Both types are transmitted by sexual contact, through blood, and from mother to child, and they appear to cause clinically indistinguishable AIDS. However, it seems that HIV-2 is less easily transmitted, and the period between initial infection and illness is longer in the case of HIV-2.

HIV Types, Subtypes & Groups
Subtypes of HIV-1
This diagram illustrates the different levels of HIV classification. Each type is divided into groups, and each group is divided into subtypes and CRF's.

The strains of HIV-1 can be classified into three groups : the "major" group M, the "outlier" group O and the "new" group N. These three groups may represent three separate introductions of simian immunodeficiency virus into humans.

Group O appears to be restricted to west-central Africa and group N - discovered in 1998 in Cameroon - is extremely rare. More than 90% of HIV-1 infections belong to HIV-1 group M. Within group M there are known to be at least nine genetically distinct subtypes (or clades) of HIV-1. These are subtypes A, B, C, D, F, G, H, J and K.

Occasionally, two viruses of different subtypes can meet in the cell of an infected person and mix together their genetic material to create a new hybrid virus (a process similar to sexual reproduction, and sometimes called "viral sex"). Many of these new strains do not survive for long, but those that infect more than one person are known as "circulating recombinant forms" or CRF's . For example, the CRF A/B is a mixture of subtypes A and B.

The classification of HIV strains into subtypes and CRF's is a complex issue and the definitions are subject to change as new discoveries are made. Some scientists talk about subtypes A1, A2, A3, F1 and F2 instead of A and F, though others regard the former as sub-subtypes.

Locations of subtypes and CRF's
The HIV-1 subtypes and CRF's are very unevenly distributed throughout the world, with the most widespread being subtypes B and C.

Subtype C is largely predominant in southern and eastern Africa, India and Nepal. It has caused the world's worst HIV epidemics and is responsible for around half of all infections.

Historically, subtype B has been the most common subtype/CRF in Europe, the Americas, Japan and Australia. Although this remains the case, other subtypes are becoming more frequent and now account for at least 25% of new infections in Europe.

Subtype A and CRF A/G predominate in west and central Africa, with subtype A possibly also causing much of the Russian epidemic. Subtype D is generally limited to east and central Africa; A/E is prevalent in south-east Asia, but originated in central Africa; F has been found in central Africa, south America and eastern Europe; G and A/G have been observed in western and eastern Africa and central Europe.

Subtype H has only been found in central Africa; J only in central America; and K only in the Democratic Republic of Congo and Cameroon.

It is almost certain that new HIV genetic subtypes and CRF's will be discovered in the future, and indeed that new ones will develop as virus recombination and mutation continue to occur. The current subtypes and CRF's will also continue to spread to new areas as the global epidemic continues.

What difference does the subtypes make?
The study have shown that some subtype are more harmful than others. Like, subtype D is more virulent because it is more effective at binding to immune cells.

An earlier study in 1999, found that women infected with subtype C, D or G were more likely to develop AIDS within five years of infection than those infected with subtype A.

It has been observed that certain subtypes/CRF's are predominantly associated with specific modes of transmission. In particular, subtype B is spread mostly by homosexual contact and intravenous drug use (essentially via blood), while subtype C and CRF A/E tend to fuel heterosexual epidemics (via a mucosal route).

Until about 1994, it was generally thought that individuals do not become infected with multiple distinct HIV-1 strains. Since then, many cases of people co-infected with two or more strains have been documented.

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