Viruses are known to cause cancer in animals but their role in the genesis of human cancer is a subject of debate. It is difficult for scientists to prove that viruses cause human cancer, because direct experiments on human beings are not allowed. Some of the evidence suggests the causative role of viruses in human cancers. About 90 per cent cases of nasopharyngeal carcinoma have the antigens of Epstein-Barr virus (EBV). There is a link between EBV and Burkitt’s lymphoma. Studies have suggested an association between EBV and the testicular tumours. Epstein-Barr virus is also associated with leiomyoma in children. Similarly, Hepatitis B and Hepatitis C viruses are known to enhance the risk of hepatocellular carcinoma. Human immunodeficiency virus (HIV) is associated with non-Hodgkin’s lymphoma, Kaposi’s sarcoma and cancers of the cervix & anus. The findings of DNA sequences of human herpes virus-8 (HHV-8) in Kaposi’s sarcoma and multiple myeloma support the causative role of herpes virus in cancer. Sexually transmitted human papilloma virus (HPV) is a major risk factor of cervical & anal cancers.
[Virus is a sub microscopic entity consisting of a molecule of DNA or RNA covered by a protective protein coating called capsid. It has been observed that the DNA strands of viruses insert directly into one of the chromosomes of the animal cell, causing mutations that can lead to the development of cancer. Some of the RNA viruses are found to carry an enzyme called reverse transcriptase that transcribes RNA to DNA. The strands of transcribed DNA then insert into the chromosomes of the animal cell, causing mutations in the genes]