Cancer and Healthy

Some cancers can be caused by infection with pathogens.^[24] 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.^[25] 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.

Advances in cancer research have made a vaccine designed to prevent cancer available. In 2006, the US FDA approved a human papilloma virus vaccine, called Gardasil. The vaccine protects against four HPV types, which together cause 70% of cervical cancers and 90% of genital warts. In March 2007, the US CDC Advisory Committee on Immunization Practices (ACIP) officially recommended that females aged 11-12 receive the vaccine, and indicated that females as young as age 9 and as old as age 26 are also candidates for immunization.

In addition to viruses, researchers have noted a connection between bacteria and certain cancers. The most prominent example is the link between chronic infection of the wall of the stomach with Helicobacter pylori and gastric cancer.^[26][27] Although only a minority of those infected with Helicobacter go on to develop cancer, since this pathogen is quite common it is probably responsible for the majority of these cancers.^[28]