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In an individual infected with HIV, there are as many as 10 billion viral particles. Based on the observed genetic diversity in clinical isolates, it is estimated that there are 100 million distinct genomexs per individual. If one does the math, this means that there are only 100 viral particles that have exactly the same genome. This has led to the term quasispecies to describe the population of viruses.

HIV reverse transcriptase - the protein that converts RNA into DNA, is an error prone enzyme. It makes a mistake in approximately 1 out of every 1700 nucleotides that it copies. As a result, there is a very high chance that the progeny of a virus will have small changes in its genetic makeup.

This incredible amount of genetic diversity is one of the reasons why viruses are able to evolve so rapidly to changing conditions such as new hosts or antiviral therapies. By having such a broad spectrum of genotypes, the virus ensures that some subpopulation is able to adapt. Then, as the resistant strains propogate, a new quasispecies is built. The adaptive advantage conferred by genetic diversity has cleverly been called survival of the flattest.

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