Infectious causes of cancer
[4] Parasitic infections strongly associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).
[8] Over half of the world's population is colonized with H. pylori and it is estimated that H. pylori-positive patients have a 1-2% risk of developing distal gastric cancer.
[9] Genomic instability through various means such as DNA damage and epigenetic modifications[10] appear to be the basic causes of sporadic (non-familial) cancer.
HPV is well known for causing genital warts and essentially all cases of cervical cancer, but it can also infect and cause cancer in several other parts of the body, including the esophagus larynx, lining of the mouth, nose, and throat, anus, vulva, vagina, and penis.
Closely related to human T-cell leukemia virus, is another deltaretrovirus, bovine leukemia virus (BLV), which recently has met the expected criteria to accept a possible infectious agent causation of breast cancer, using sensitive PCR methods to detect BLV, and having samples from women with breast cancer compared to a control sample of women with no history of breast cancer.
After HIV destroys the immune system, the body is no longer able to control these viruses, and the infections manifest as cancer.
In addition, the high-risk HPV E6 and E7 oncoproteins can each independently induce genomic instability in normal human cells.
[34] As reviewed by Takeda et al.,[31] HCV and HBV cause carcinogenic DNA damage and genomic instability by a number of mechanisms.
HBV, and especially HCV, cause chronic inflammation in the liver, increasing reactive oxygen species (ROS) formation.
The most prominent example is the link between chronic infection of the wall of the stomach with Helicobacter pylori and gastric cancer.
[43] The mechanism by which H. pylori causes cancer may involve chronic inflammation, or the direct action of some of its virulence factors, for example, CagA has been implicated in carcinogenesis.
[48] In addition, oxidative stress, with high levels of 8-OHdG in DNA, also affects genome stability by altering chromatin status.
[51] As reviewed by Santos and Ribeiro[52] H. pylori infection is associated with epigenetically reduced efficiency of the DNA repair machinery, which favors the accumulation of mutations and genomic instability as well as gastric carcinogenesis.
[55] Colibactin is a genotoxin produced by Escherichia coli infection that can cause colorectal cancer to develop.
[56] One meta-analysis of serological data comparing prior Chlamydia pneumoniae infection in patients with and without lung cancer found results suggesting prior infection was associated with a slightly increased risk of developing lung cancer.
[60] Inflammation triggered by the worm's eggs appears to be the mechanism by which squamous cell carcinoma of the bladder is caused.
[60] Distomiasis, caused by parasitic liver flukes, is associated with cholangiocarcinoma (cancer of the bile duct) in East Asia.
[60] Malaria is associated with Burkitt's lymphoma in Africa, especially when present in combination with Epstein-Barr virus, although it is unclear whether it is causative.
Cysticercus fasciolaris, the larval form of the common tapeworm of the cat, Taenia taeniaformis, causes cancer in rats.
This isolated case has no substantive bearing on public health but is interesting for being "a novel disease mechanism that links infection and cancer.
