Approximately 15% of tumours worldwide can be attributed to viruses; the majority of these are represented by hepatocellular and cervical carcinoma and are the sequelae of infection by two DNA viruses, the hepatitis B virus and the human papillomavirus respectively. Other viruses directly linked to human tumours include:
♦ Human immunodeficiency virus (HIV).
♦ Cytomegalovirus (CMV). Viruses indirectly linked include:
Although viral infection plays a significant role in the initial step towards carcinogenesis, acquisition of malignant phenotype requires additional genetic alterations.
Most tumour viruses are ubiquitous; prevalence of infection is much higher than the incidence of the respective form of tumour, and development of associated tumours requires many years of infection. This latent period may last decades. Other co-factors are necessary for development of virally-linked tumours, including genetic, immunological, hormonal, and environmental factors.
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