Some helminths cause cancer, while others protect against it
Helminths that have been identified as being carcinogenic include the fish-borne trematodes Opisthorchis viverrini and Clonorchis sinensis and the blood fluke Schistosoma haematobium, all of which are categorised by the International Agency for Research on Cancer as Group 1 biological carcinogens.  
The helminths used for therapeutic purposes (NA, TT, TS and HD) are not carcinogenic (see Helminthic therapy safety), but one helminthic therapist has suggested that, since some solid mass tumours like to surround themselves with regulatory immune cells, it may be unwise to increase this cell population in patients with a history of solid mass tumours. 
The protective role of helminths
Helminths may be able to elicit anti-tumor immune responses that can lead to protection from tumorigenesis, or even to cancer regression.
Helminth infection may limit the growth and dispersion of tumors.
- The immune response to Hymenolepis nana in mice decreases tumorigenesis induced by 7,12 dimethylbenz-anthracene
Helminth infection may reduce the risk of colitis-associated tumour formation.
- Helminth-derived molecules inhibit colitis-associated colon cancer development through NF-kB and STAT3 regulation
- Gastrointestinal Nematode-Derived Antigens Alter Colorectal Cancer Cell Proliferation and Migration through Regulation of Cell Cycle and Epithelial-Mesenchymal Transition Proteins
Helminth infection may alter inflammatory responses to H. pylori and thus affect the progression of gastritis to gastric atrophy, dysplasia, and cancer.
- Intestinal helminthiasis in Colombian children promotes a Th2 response to Helicobacter pylori: possible implications for gastric carcinogenesis
The high prevalence of helminth infections in Sub-Saharan Africa may be responsible for a reduced risk of gastric adenocarcinoma.
- The impact of Helicobacter pylori and intestinal helminth infections on gastric adenocarcinoma and inflammatory bowel disease in Sub-Saharan Africa -- Full text | PDF
Helminths and their products may potentially treat cancer
Helminths exert antitumor effects via several mechanisms of action.
The following anecdotal report suggests a positive contribution by NA to treatment in one case of cancer.
- And an update three years later.
- Re-innoculated myself with 10 N. americanus (NA) on July 13th 2023, after successfully hosting NA back in late 2019 and into 2022. My goal back in 2019 and now remains the same. Combatting my hereditary condition known as FAP (familial adenomatous polyposis) which fueled my diagnosis of Stage IV colorectal cancer five years ago, at the age of 30. Within the first 3-4 months; my body, spirit, overall health - and cancer prognosis - sobered up! I was also put on (just before inoculation) a last ditch effort immunotherapy that had "little hope of prolonged life". This PD-1 inhibitor treatments, in my opinion, might have worked synergistically with the NA response to my immuno profile. I credited the NA on two things; 1) helping my body repair DNA and address inflammation from my flawed inherited genes, & 2) promote pathways that helped go after my tumors littered throughout my lymph nodes, liver, diaphragm, and colon. Specifically interleukin pathways and the release of high amounts of eosinophils (EOs) which are known to interrupt colorectal cancer cells. This also helped me during my first reoccurrence with the disease in early 2021, while on an experimental trial in which I was about the best (of very few) positive responses. I killed off my previous colony in favor of a highly recommended protocol involving a common ingredient in dog dewormer while in remission in October of 2022, my last NA innoculation before killing off the NA was in April 2022. Fast forward to now (2023) I was recently re-diagnosed with cancer after another year of remission. My last CT scan showed some growth in my lymph nodes with no additional spread. Hoping this works with my last standard of care treatment. 
Worm-derived molecules could be potential candidates for anti-cancer drugs.
- Anti-Tumor Effect of Marshallagia marshalli Somatic Antigen on Inhibition Cell Growth of K562 -- Full text | PDF
- Immunomodulatory action of excretory-secretory products of Angiostrongylus cantonensis in a mouse tumour model
- Hookworm exposure decreases human papillomavirus uptake and cervical cancer cell migration through systemic regulation of epithelial-mesenchymal transition marker expression -- Full text | PDF
- The helminth-derived peptide GK-1 induces an anti-tumoral CD8 T cell response associated with downregulation of the PD-1/PD-L1 pathway
Infection with other microorganisms may also help to reduce cancer risk
BCG vaccination in infancy confers a survival advantage for melanoma patients, and vaccination of adults against yellow fever may have a similar effect.
- The biography of the immune system and the control of cancer: from St Peregrine to contemporary vaccination strategies
Infection with the feline parasite, Toxoplasma gondii, stimulates the body to produce natural killer cells and cytotoxic T cells, which wage war on cancer cells.
Rather than the presence of infectious microorganisms increasing cancer risk, a lack of them may be the greater problem.
Anthelmintic drugs may be used in cancer treatment
This does not mean that terminating any helminths that a cancer patient may be hosting will help to treat their cancer.
When an anthelmintic drug such as mebendazole (Vermox) is used in cancer treatment, it is not because of its worm-killing effects, but because of its ability to act directly on cancers, blocking tumor growth and spread, inducing apoptosis, and increasing sensitivity to other anti-cancer therapies. 
However, the use of an anthelmintic as part of a patient’s cancer treatment would prevent that patient from hosting helminths while they remain on that treatment.
When a helminth, itself, gets cancer
This rare case in which a man died after a tapeworm inside him developed cancer, involved a helminth that is not used in therapy.
For a discussion about this case, see this support group thread.