Infection with hepatitis C virus (HCV) is a global public health problem. It is estimated that about 71 million people, are infected with the virus worldwide. Typically, HCV will establish a chronic infection that does not clear without intervention, leading to progressive liver disease and increased risk of hepatocellular cancer. It is now recognised that HCV infection is one of the leading causes of serious liver disease in many countries and is a considerable burden on healthcare services. Previously, treatment consisted of a long course of interferon and ribavirin which was often poorly tolerated due to side effects, and had a low overall success rate. Although there are now new drugs available (called direct-acting antivirals; DAAs) that do not require the use of interferon and give much higher cure rates even with reduced length of treatment, resistance to these drugs has been demonstrated which may limit their use in the future.
As well as being an important clinical pathogen, HCV is also interesting in terms of fundamental research as it is one of the most variable pathogens known to infect humans. It has been divided into 8 genotypes based on diversity at the nucleotide sequence level but there also many sub-types of each genotype. Even within an infected person, the virus is highly variable and exists as quasispecies, which is a population of diverse but closely-related variants. The extent of variability will depend on many factors; the amount of circulating virus, the genetics of the infected host and other external factors.