Many viruses are associated with certain types of cancer, but some bacteria and parasites also increase cancer risk. Besides Helicobacteria pylori and some parasites, most of these associations remain controversial or contested, however.
Some of these microbes promote cancer by directly causing mutations in cells, while others cause chronic inflammation locally, a well-known risk factor for cancer. There are other mechanisms too, for example reducing clearance of carcinogens.
Helicobacteria
Helicobacter pylori is a common chronic infection of the stomach, which increases the risk of peptic ulcers. It also increases the risk of stomach cancer and some much rarer gastric lymphomas. In some areas up to 80% of gastric cancers may be caused by H. pylori. Peculiarly it appears to decrease the incidence of esophageal cancer and cancer of the gastric cardia.
Lesser-known Helicobacter bilis and H. hepaticus have been implicated in liver cancer and possibly colon cancer. Helicobacter heilmannii may increase the risk of stomach cancer and MALT lymphoma similar to H. pylori. Helicobacter species have also been linked to biliary tract cancer (including gallbladder cancer).
Helicobacter spp. appears to also further increase the risk of liver cancer in people with hepatitis B and C.
Mycoplasma
Mycoplasmas were first associated with cancer back in the 1960s. Later many of those associations were found to have been caused by contamination.
More recently, however, new studies have surfaced, linking Mycoplasma hyorhinis in particular to many cancers, especially gastric carcinoma (stomach cancer). Another association that has been studied by many researchers is that between several mycoplasma species and prostate cancer.
Other Bacteria
Several bacteria and bacteria-like organisms such as Borrelia burgdorferi (Lyme disease), Chlamydia psittaci (psittacosis) and Campylobacter jejuni (which causes food poisoning) have been associated with lymphomas.
Chlamydia pneumoniae and Mycobacterium tuberculosis may increase the risk of lung cancer. Salmonella typhi appears to be a strong risk factor for gallbladder cancer and other biliary tract cancers. Some chronic infections appear to increase the risk of squamous cell skin cancer.
Some cases of colorectal cancer have been connected to Streptococcus bovis and possibly E.coli, though the causal relationship is unclear. Streptococcus anginosus may be connected to esophageal cancer and possibly oral cancer. Several other bacteria have been putatively linked to oral cancer, as well.
Parasites
The first reports about a possible association with some parasites and cancer surfaced in the year 1900. The earliest reports concerned Schistosoma haematobium infections in bladder cancer and Opistorchis felineus in liver cancer.
Besides bladder cancer, everal Schistosoma (bilharzia) species have been implicated in liver cancer and gastrointestinal cancers. Opistorchis felineus, Opistorchis viverrini and Clonorchis sinensis are strongly associated with cholangiocarcinoma in the parts of the world where these liver flukes are endemic.
Many other associations for Schistosoma species and other parasites have been suggested, but they are difficult to research, as parasites are mostly a problem of the developing world, where people often have multiple chronic infections and other health problems.
Helminths may reduce risk of gastric cancer by Helicobacter pylori by reducing the inflammatory response, though this has not been fully confirmed. Blastocystes hominis appears to enhance the growth of colorectal cancer.
The Relevance of Bacteria and Parasites in Cancer Treatment
Most bacteria and many parasites are relatively easy to treat to reduce the risk of cancer, though some chronic infections like borrelia, mycoplasma and chlamydia can be difficult to eradicate. H. pylori eradication may even regress an already existing gastric lymphoma, but in most cases antibiotic treatment is not thought to affect the outcome of cancer, even if originally caused by a bacterial infection.
Many quack treatments are claimed to "cure cancer" by killing bacteria or parasites, often by supposedly emitting some sort of electromagnetic fields. There is no evidence to support the use of any of these machines/devices in the treatment of cancer (or any other purpose) - unfortunately cancer treatment is a field rife with quackery.
Interestingly, some bacteria and bacterial toxins may also be used as a treatment for cancer, such as Coley's toxins, a type of immunotherapy invented back in the 1800s.
References
zur Hausen, Harald. "Infections Causing Human Cancer." Wiley-WCH. 2006.
Mager DL. "Bacteria and cancer: cause, coincidence or cure?" A review. J Transl Med. 2006 Mar 28;4:14.
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