The HIV (Human immunodeficiency virus) is a retrovirus that gradually destroys the immune system and ultimately leads to AIDS, is famously the most genetically diverse viral pathogen known, specially in Africa where HIV is also rapidly mutating. Indeed, HIV exists under many different versions like members of a large family, they are different from, but related to each other.

By sequencing the viral genomes (genes), researchers have been able to map out the family tree of HIV. At the root of the tree, there are three groups called M, N and O, group M being responsible for the current AIDS pandemic. Group M is split into nine genetic subtypes, also called nine clades (designated A through K, with no E or I). The original definition of clades was based on short genomic sequences, mostly within the HIV envelope protein (Env: gp160).

These nine clades have uneven geographic distribution patterns. Clade C circulates in South Africa, India and parts of China. Clade A and D are common in East Africa and clade B is common in North & South America and Western Europe. Looking at the global numbers, it emerges that four clades (A, B, C and D) plus two recombinant forms called CRFs 01 and 02 (both of which are about 70% clade A) account for over 90% of all infections worldwide. From this perspective, diversity can be mostly limited to 4 key major clades, plus small contributions from the non-A segments of these two CRFs. According to the statistics, clade C represents the world dominant HIV (>50%).

HIV attaches itself to the target host cell using a harpoon-like surface protein called gp160. This protein spears the host cell's membrane, drawing them together so that the virus can fuse with the host cell.

Once attached, the virus penetrates the cell and commandeers the cell's machinery. Then it rapidly replicates itself.

What makes HIV-1 so lethal? It targets the most central cell of the immune system, the CD4+ T cells which produce the IL-2 cytokine, a key messenger for immune cells. These cells usually coordinate the cellular and humoral responses that are directed to thwart the pathogen (HIV). When the number of such CD4+ T cells decreases significantly over time, the amount of IL-2 becomes also too low for an efficient immune attack orchestration. Consequently, HIV as well as other pathogens evade the activity of the immune system, leaving the host vulnerable to disease.

HIV proves itself an elusive target because it:
• reproduces itself at an extraordinary rate (several billion new virus particles are created daily)
• mutates rapidly: as it reproduces itself, it makes mistakes that produce new virus particles that are slightly different; these differences make the virus harder to target by the immune system.