and Central Africa, infecting for centuries human populations carrying the genetic polymorphism in question. This has prompted scientists to suggest that the protracted persistence of HCV genotypes 1 and 4 in people of African origin is an evolutionary adaptation of HCV over many centuries to these populations’ immunogenetic responses. Infection with one genotype does not confer immunity against others, and concurrent infection with two strains is possible. In most of these cases, one of the strains outcompetes the other in a short time. This finding may be useful in treatment, in replacing strains non-responsive to medication with others easier to treat. Recombination When two viruses infect the same cell, genetic recombination may occur. Although infrequent, HCV recombination has been observed between different genotypes, between subtypes of the same genotype and even between strains of the same subtype. Epidemiology Hepatitis C virus is predominantly a blood-borne virus, with very low risk of sexual or vertical transmission. Because of this mode of spread the key groups at risk are intravenous drug users (IDUs), recipients of blood products and sometimes patients on haemodialysis. Common setting for transmission of HCV is also intra-hospital (nosocomial) transmission, when practices of hygiene and sterilization are not correctly followed in the clinic. A number of cultural or ritual practices have been proposed as a potential historical mode of spread for HCV, including circumcision, genital mutilation, ritual scarification, traditional tattooing and acupuncture. It has also been argued that given the extremely prolonged periods of persistence of HCV in humans, even very low and undetectable rates of mechanical transmission via biting insects may be sufficient to maintain endemic infection in the tropics, where people receive large number of insect bites. Evolution Identification of the origin of this virus has been difficult but genotypes 1 and 4 appear to share a common origin. A Bayesian analysis suggests that the major genotypes diverged about 300–400 years ago from the common ancestor virus. The minor genotypes diverged about 200 years ago from their major genotypes. All of the extant genotypes appear to have evolved from genotype 1 subtype 1b. A study of genotype 6 strains suggests an earlier date of evolution: approximately 1,100 to 1,350 years Before Present. The estimated rate of mutation was 1.8 × 10−4. An experimental study estimated the mutation rate at 2.5–2.9 × 10−3 base substitutions per site per year. This genotype may be the ancestor of the other