Sources of
ionizing radiation, such as
radon gas, can cause cancer. Prolonged exposure to
ultraviolet radiation from the
sun can lead to
melanoma and other skin malignancies.
[11]Non-ionizing radio frequency radiation from
mobile phones and other sources has also been proposed as a cause of cancer, but there is little evidence of such a link.
[12] Nevertheless, a few experts caution against prolonged exposure based on the
precautionary principle.
[13]Viral or bacterial infection
Some cancers can be caused by
infection with
pathogens.
[14] Many cancers originate from a
viral infection; this is especially true in animals such as
birds, but also in
humans, as viruses are responsible for 15% of human cancers worldwide. The main viruses associated with human cancers are
human papillomavirus,
hepatitis B and
hepatitis C virus,
Epstein-Barr virus, and
human T-lymphotropic virus. Experimental and epidemiological data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage.
[15] The mode of virally-induced tumors can be divided into two, acutely-transforming or slowly-transforming. In acutely transforming viruses, the virus carries an overactive oncogene called viral-oncogene (v-onc), and the infected cell is transformed as soon as v-onc is expressed. In contrast, in slowly-transforming viruses, the virus genome is inserts near a proto-oncogene in the host genome. The viral
promoter or other transcription regulation elements then cause overexpression of that proto-oncogene. This induces uncontrolled cell division. Because the site of insertion is not specific to proto-oncogenes and the chance of insertion near any proto-oncogene is low, slowly-transforming viruses will cause tumors much longer after infection than the acutely-transforming viruses.
Hepatitis viruses, including
hepatitis B and
hepatitis C, can induce a chronic viral infection that leads to
liver cancer in 0.47% of
hepatitis B patients per year (especially in Asia, less so in North America), and in 1.4% of
hepatitis C carriers per year. Liver cirrhosis, whether from chronic viral hepatitis infection or alcoholism, is associated with the development of
liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of
liver cancer development. Worldwide,
liver cancer is one of the most common, and most deadly, cancers due to a huge burden of
viral hepatitis transmission and disease.
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