In common with many RNA viruses, the hepatitis C virus mutates at a very high rate. This means that in any infected patient there are multiple different viruses, each differing by a few nucleotides, and one or two amino acids – in other words, the virus exists as a population of closely related but different viral species (quasispecies). Over time some of these different viral species are more successful and become dominant, so that the viral population changes. It is estimated that the dominant viral sequence changes every few weeks. These changes may help the hepatitis C virus to avoid the immune system. Since the genomic sequence of the hepatitis C virus changes every few weeks it is not surprising to find that every patient is infected with a slightly different ‘viral
cocktail’ and no two people will have an identical viral population. However, people who are initially infected with the same virus will develop a viral population that is related, and by examining the sequence of the viruses one can examine the possibility that two people were infected from a common source. These types of analysis are of obvious value in investigating iatrogenic and other outbreaks.