Parasites Classified as Cancer-Causing Agents

Table of Contents

1. What the Presentation Said
2. How the International Agency for Research on Cancer Classifies Cancer Causes
3. The Three Parasites in Group 1
4. Parasites in Groups 2A, 2B and 3
5. Parasites That Are Not on the List
6. How Many Cancers Infections Cause
7. How These Parasites Are Thought to Cause Cancer
8. What Control Programmes Report
9. How the Claim Compares With the Research
10. What Is Not Known
11. Connections
12. Key Research Papers
13. Featured Videos

What the Presentation Said

The presentation was a 2026 webinar on cancer and parasites given by the medical director of an integrative cancer clinic. On this topic the speaker made several linked points:

• Parasites carry toxins and other germs, creating what the speaker called a “toxic sink”, and in a person with cancer, parasites are part of the tumour microenvironment (the cells, blood vessels, immune cells and other material that surround a tumour).
• Parasitic infection that contributes to cancer starts with infection that promotes inflammation; the speaker described cancer as an inflammatory disease.
• The next step, in the speaker's account, is immune dysregulation: the immune system fails to detect the parasites and other microbes, which makes a person more susceptible to tumours.
• The speaker then described “parasite-driven oncogenesis” (oncogenesis means the formation of cancer), listing four mechanisms on a slide: chronic inflammation, oxidative stress, immune suppression and epigenetic change (changes in which genes are switched on or off without changes to the DNA sequence itself). In the speaker's words, parasites change the “terrain”, cancer genes are activated, oxidative stress creates more mutations, and chronic inflammation leads to DNA damage.

The presentation did not name any specific parasite as an official cause of cancer and did not refer to the International Agency for Research on Cancer classifications. It cited no study for this part of the talk. The rest of this page looks at what the official classification bodies and the published research say about which parasites cause cancer in people, and how.

How the International Agency for Research on Cancer Classifies Cancer Causes

The International Agency for Research on Cancer Monographs programme brings together panels of independent scientists who review all the published evidence on a possible cause of cancer — a chemical, a habit, a job, a type of radiation or an infection. The panel then places it in one of four groups. The groups describe how certain the evidence is that something can cause cancer, not how strong the risk is or how many cases it causes.

• Group 1 — Carcinogenic to humans. The evidence that it causes cancer in people is sufficient. As of the International Agency for Research on Cancer list updated July 2026 (Volumes 1–142), 135 agents are in this group.
• Group 2A — Probably carcinogenic to humans. 98 agents.
• Group 2B — Possibly carcinogenic to humans. 329 agents.
• Group 3 — Not classifiable as to its carcinogenicity to humans. 498 agents. This means the evidence was too thin or too mixed to decide either way. It does not mean the agent was shown to be safe.

The International Agency for Research on Cancer list itself carries two warnings that matter here. First, “Significant new information might support a different classification.” Second, “For agents that have not been classified, no determination of non-carcinogenicity or overall safety should be inferred.” In other words, a parasite missing from the list has simply not been formally reviewed, or was reviewed and placed in Group 3.

The same classification system covers far more than parasites. Infections in Group 1 include the bacterium Helicobacter pylori (stomach cancer), hepatitis B and C viruses (liver cancer), human papillomavirus (cervical and other cancers), Epstein-Barr virus, human T-cell lymphotropic virus type 1, Kaposi sarcoma herpesvirus and HIV-1, among others.

The Three Parasites in Group 1

Only three parasites are in Group 1. All three are flukes — flat worms, also called trematodes, that spend part of their life in freshwater snails. The working group that met in 2009 for Volume 100B confirmed all three as causes of cancer in people (Bouvard and colleagues, 2009); the current list shows each with Volumes 61 and 100B.

Schistosoma haematobium — bladder cancer

Schistosoma haematobium is a blood fluke. People are infected when larvae in fresh water burrow through the skin. The adult worms live in the veins around the bladder, and their spiked eggs lodge in the bladder wall. The International Agency for Research on Cancer lists it as having sufficient evidence for cancer of the urinary bladder. The cancer it is linked to is mainly squamous cell carcinoma, a type of bladder cancer that is relatively uncommon in places without the parasite (Yosry, 2006). More detail is on the page Schistosoma and Bladder Cancer.

Opisthorchis viverrini — bile duct cancer

Opisthorchis viverrini is a liver fluke found mostly in Southeast Asia, especially the Mekong basin. People are infected by eating raw or undercooked freshwater fish that carry the larvae. The adult worms live in the bile ducts. The International Agency for Research on Cancer lists it as having sufficient evidence for cancer of the bile duct (cholangiocarcinoma). A 2023 overview estimated that 67.3 million people in Southeast Asia are at risk of O. viverrini infection, and named northeast Thailand as the world's hot spot of liver fluke infection and bile duct cancer (Khuntikeo and colleagues, 2023).

Clonorchis sinensis — bile duct cancer

Clonorchis sinensis, the Chinese liver fluke, is closely related to O. viverrini and is found mainly in eastern Asia. It is caught the same way, from raw or undercooked freshwater fish, and is also listed for cancer of the bile duct. A 2024 review describes chronic infection with both flukes as linked to liver and bile duct complications, “most importantly cholangiocarcinoma” (Qian and colleagues, 2024). Counting all three related liver flukes, about 700 million people worldwide are estimated to be at risk of infection (Khuntikeo and colleagues, 2023). The two liver flukes are covered in depth on Liver Flukes and Bile Duct Cancer.

Several reviews describe these three flukes as the only parasites with sufficient human evidence (Vennervald and Polman, 2009; Feng and Cheng, 2017; van Tong and colleagues, 2017).

Parasites in Groups 2A, 2B and 3

Four more parasites appear on the International Agency for Research on Cancer list in lower groups. Each placement reflects weaker or less consistent evidence.

Malaria (Plasmodium falciparum) — Group 2A, probably carcinogenic

The International Agency for Research on Cancer lists “Malaria (caused by infection with Plasmodium falciparum in holoendemic areas)” in Group 2A, from Volume 104 (evaluated 2012, published 2014). “Holoendemic” means areas where almost every child is infected, again and again, from early life. The cancer is endemic Burkitt lymphoma, a fast-growing cancer of white blood cells that occurs in children in parts of Africa. The working group's published summary appeared in 2012 (Bouvard and colleagues, 2012). In the International Agency for Research on Cancer cancer-site list, Epstein-Barr virus has sufficient evidence for Burkitt lymphoma and malaria has limited evidence. A 2017 review explains that malaria by itself does not appear to cause cancer; endemic Burkitt lymphoma also needs the transforming events set off by Epstein-Barr virus (van Tong and colleagues, 2017). The full story is on Malaria and Burkitt Lymphoma.

Schistosoma japonicum — Group 2B, possibly carcinogenic

Schistosoma japonicum, the blood fluke of East Asia, has been in Group 2B since Volume 61 (1994). The International Agency for Research on Cancer cancer-site list records limited evidence in humans for cancers of the colon, rectum, liver and bile duct. Reviews describe a long-reported link with colorectal cancer that “remains understudied” (Hamid, 2019), and a link with liver cancer that one 2025 review calls “circumstantial”. That review lists possible contributing factors — inflammation and scarring around trapped eggs, effects of egg proteins, and co-infection with hepatitis viruses — and concludes that more studies are needed before infection with S. japonicum alone can be labelled a liver carcinogen (Jain, 2025).

Schistosoma mansoni — Group 3, not classifiable

Schistosoma mansoni, a blood fluke whose adult worms live in the veins around the intestine, is in Group 3 (Volume 61, 1994). An earlier review called the human evidence “inadequate”, while noting that S. mansoni might still be linked to liver cancer by worsening the effects of hepatitis B and C (Yosry, 2006). A 2021 review argues that case reports, together with newer data from animal models and cell culture experiments, suggest the parasite may predispose people to liver and colorectal cancer — but it remains an argument for further study, not a reclassification (von Bülow and colleagues, 2021).

Opisthorchis felineus — Group 3, not classifiable

Opisthorchis felineus is the third liver fluke, found mainly in Russia and eastern Europe (Qian and colleagues, 2024). It is in Group 3 (Volume 61, 1994). Its biology closely resembles the two Group 1 liver flukes, and surgical and experimental reports suggest it might raise the risk of bile duct cancer. However, a review of official Russian statistics for 2011–2013 across 83 regions found no association between the reported rate of O. felineus infection and the rate of liver and intrahepatic bile duct cancers (correlation r = 0.20, p = 0.07), and stated that no association between infection and bile duct cancer had yet been demonstrated (Fedorova and colleagues, 2017). Researchers continue to ask whether the three liver flukes really differ in their ability to cause cancer, and why (Pakharukova and Mordvinov, 2022).

Parasites That Are Not on the List

A search of the current International Agency for Research on Cancer list (Volumes 1–142, updated July 2026) found no entry for Toxoplasma, Trichomonas, Strongyloides, Trypanosoma, Echinococcus, Cryptosporidium, Giardia, Fasciola or the pork tapeworm Taenia. As the International Agency for Research on Cancer itself notes, not being listed is not proof of safety — it means no formal evaluation has placed them in any group.

Research on several of these is ongoing and mixed:

• A 2017 review describes Trypanosoma cruzi, the cause of Chagas disease, as having a “dual role”, with both cancer-promoting and anti-cancer properties reported in research (van Tong and colleagues, 2017).
• The same review notes observations suggesting that the roundworm Strongyloides stercoralis may be a co-factor in T-cell lymphomas caused by the virus HTLV-1 (which itself is in Group 1).
• Toxoplasma has been studied both as a possible risk factor for brain tumours and, in mice, as a tool against tumours — see Toxoplasma and Brain Tumors.

These and other weaker links are gathered on Other Suspected Links Between Parasites and Cancer, which also covers studies in which worm infections appeared protective.

How Many Cancers Infections Cause

Researchers at the International Agency for Research on Cancer have repeatedly estimated how much of the world's cancer is caused by infections. They use a measure called the population attributable fraction — the share of cases that would not have happened if the infection had been absent.

• 2008: Of 12.7 million new cancer cases, about 16.1% — around 2 million — were attributed to infections. The share was 22.9% in less developed countries and 7.4% in more developed countries, ranging from 3.3% in Australia and New Zealand to 32.7% in sub-Saharan Africa. H. pylori, hepatitis B and C and human papillomavirus accounted for 1.9 million of these cases (de Martel and colleagues, 2012).
• 2018: An estimated 2.2 million infection-attributable cancer cases, again led by H. pylori (810,000), human papillomavirus (690,000), hepatitis B (360,000) and hepatitis C (160,000). Rates were highest in eastern Asia and sub-Saharan Africa (de Martel and colleagues, 2020).
• 2024: An estimated 2.3 million new cancer cases — 12% of all cancers — were attributed to 12 Group 1 infectious agents (Rumgay and colleagues, 2026).

The parasites' share

The 2024 analysis is the first of these in which the parasite figures can be read directly from the published tables:

• About 5,900 cases of squamous cell carcinoma of the bladder were attributed to Schistosoma haematobium — 33% of the roughly 18,000 such cases worldwide.
• About 3,800 cases of intrahepatic cholangiocarcinoma (bile duct cancer inside the liver) were attributed to O. viverrini and C. sinensis together — 2% of about 160,000 cases.

Put together, the three Group 1 parasites accounted for roughly 9,700 of the estimated 2.3 million infection-related cancers in 2024 — well under 1%. Viruses and the bacterium H. pylori account for almost all of the rest (Rumgay and colleagues, 2026). An earlier review made the same point: only a relatively small part of infection-related cancer is due to worms, but those infections matter because so many people are exposed (Vennervald and Polman, 2009). The burden is concentrated where the parasites live: the liver fluke cancers in Southeast and East Asia, and S. haematobium bladder cancer mainly in sub-Saharan Africa.

How These Parasites Are Thought to Cause Cancer

The mechanisms proposed in the research overlap with several items on the presentation's slide. A 2009 review summarised them as follows (Vennervald and Polman, 2009):

• Chronic inflammation is the key feature. The body's long-running immune response to the worms and their eggs keeps the tissue inflamed for years.
• Free radicals and reactive nitrogen compounds made by inflammatory cells can damage DNA and lead to genetic instability.
• Physical damage from the parasites, their eggs or the substances they secrete leads to repeated repair and cell growth in the damaged tissue. This can multiply cells that already carry DNA damage.

A later review of fluke-related cancers adds that infection is often silent for years, so it is rarely diagnosed early, and that the cell and molecular steps linking fluke infection to cancer “have yet to be defined” despite much recent research (Feng and Cheng, 2017). For S. japonicum and colorectal cancer, a review lists chronic inflammation, changes to immune responses, schistosome toxins and bacterial co-infection as likely contributors (Hamid, 2019).

These mechanisms are worked out mostly from the three Group 1 flukes, in people and animals with long-term, heavy infections. They are explained more fully on How Parasites Can Drive Cancer: The Mechanisms.

What Control Programmes Report

Because the three Group 1 parasites are infections, the cancers they cause are described in the research as preventable. A 2009 review stated that reducing infection and the risk of reinfection will reduce the later risk of cancer, and called for sustained worm control in endemic areas together with health education, especially about food-borne liver flukes (Vennervald and Polman, 2009).

• For the liver flukes, a 2024 review describes the mainstay of control as preventive chemotherapy with the antiparasitic medicine praziquantel, usually combined with education to change raw-fish eating habits, and environmental control. It names tribendimidine as a candidate drug and fish vaccines and biological control as newer ideas under study (Qian and colleagues, 2024).
• For S. haematobium, the 2024 global analysis lists preventive chemotherapy with praziquantel — especially mass drug administration in endemic areas — together with improved water, sanitation and hygiene, as the available ways to reduce the bladder cancer burden (Rumgay and colleagues, 2026).
• The authors of the Russian O. felineus review, despite finding no statistical link with cancer, concluded that the infection data supported a public-health control programme and wider access to antiparasitic treatment (Fedorova and colleagues, 2017).

These programmes aim to prevent infection, and through that, cancer, in populations where the parasites are common. They are not treatments for an existing cancer. Decisions about cancer treatment and antiparasitic medicines belong with a clinician.

How the Claim Compares With the Research

Supported by the research

• Some parasitic infections do cause cancer in people. Three flukes — Schistosoma haematobium, Opisthorchis viverrini and Clonorchis sinensis — are in the International Agency for Research on Cancer's highest category of certainty.
• Chronic inflammation, oxidative DNA damage and changes to immune responses are the main mechanisms described in reviews of these fluke cancers, which matches several items the presentation listed.

Supported only in part

• The presentation spoke of parasitic infection in general. The research evidence is specific: it applies to a handful of species, each tied to particular organs (bladder, bile ducts, and for the 2A and 2B parasites, lymph tissue, colon, rectum and liver). For most parasites there is no comparable evidence.
• The presentation linked cancer to the immune system failing to detect parasites. The reviews describe something different for the flukes: the immune system does respond, and it is the long-running inflammatory response itself that is thought to damage tissue.
• Epigenetic change was on the presentation's slide. It is studied as a possible pathway, but the reviews read for this page did not establish it as a demonstrated mechanism in parasite-related cancer in people; one states that the molecular steps are not yet defined (Feng and Cheng, 2017).

Not supported, or no research found

• The statement that parasites are part of the tumour microenvironment in cancer patients generally: no study read for this page showed parasites present in tumours outside the specific fluke-related cancers. This question is examined on Are Parasites Inside Every Tumor?
• The overall scale: published estimates attribute well under 1% of infection-related cancers worldwide to parasites, compared with most of them to viruses and H. pylori (Rumgay and colleagues, 2026).

What Is Not Known

• Whether Opisthorchis felineus causes bile duct cancer as its two close relatives do. Its classification is Group 3, and the statistical data from Russia so far show no clear link (Fedorova and colleagues, 2017).
• Whether Schistosoma japonicum and Schistosoma mansoni cause liver or colorectal cancer by themselves, or only alongside other causes such as hepatitis viruses (Yosry, 2006; Jain, 2025).
• The exact cell and molecular steps by which fluke infection turns into cancer (Feng and Cheng, 2017).
• The true size of the problem. The burden estimates depend on infection data that are patchy in many endemic regions; a 2024 review lists mapping where the liver flukes now occur and estimating their disease burden accurately as pressing research needs (Qian and colleagues, 2024).
• Whether any classification will change. The International Agency for Research on Cancer states that significant new information might support a different classification, and the malaria and S. japonicum placements rest on evaluations from 2012 and 1994.

Connections

• Cancer and Parasites
• Schistosoma and Bladder Cancer
• Liver Flukes and Bile Duct Cancer
• Malaria and Burkitt Lymphoma
• How Parasites Can Drive Cancer: The Mechanisms
• Parasites
• Schistosoma
• Clonorchis Sinensis
• Malaria
• Strongyloides
• Praziquantel Treatment
• Cholangiocarcinoma
• Bladder Cancer
• Colorectal Cancer
• Liver Cancer
• Helicobacter Pylori: Cancer Risk and Complications

Key Research Papers

1. Rumgay H, Georges D, Huang Y, Hirabayashi M, Shah R, de Martel C, Park JY, Soerjomataram I, Clifford GM. Global burden of cancer attributable to infections in 2024: a worldwide incidence analysis. The Lancet Oncology. 2026;27(10):1237-1248. PubMed PMID: 42805198
2. de Martel C, Georges D, Bray F, Ferlay J, Clifford GM. Global burden of cancer attributable to infections in 2018: a worldwide incidence analysis. The Lancet Global Health. 2020;8(2):e180-e190. PubMed PMID: 31862245
3. de Martel C, Ferlay J, Franceschi S, Vignat J, Bray F, Forman D, Plummer M. Global burden of cancers attributable to infections in 2008: a review and synthetic analysis. The Lancet Oncology. 2012;13(6):607-15. PubMed PMID: 22575588
4. Bouvard V, Baan R, Straif K, Grosse Y, Secretan B, El Ghissassi F, Benbrahim-Tallaa L, Guha N, Freeman C, Galichet L, Cogliano V; WHO International Agency for Research on Cancer Monograph Working Group. A review of human carcinogens—Part B: biological agents. The Lancet Oncology. 2009;10(4):321-2. PubMed PMID: 19350698
5. Bouvard V, Baan RA, Grosse Y, Lauby-Secretan B, El Ghissassi F, Benbrahim-Tallaa L, Guha N, Straif K; WHO International Agency for Research on Cancer Monograph Working Group. Carcinogenicity of malaria and of some polyomaviruses. The Lancet Oncology. 2012;13(4):339-40. PubMed PMID: 22577663
6. Vennervald BJ, Polman K. Helminths and malignancy. Parasite Immunology. 2009;31(11):686-96. PubMed PMID: 19825108
7. van Tong H, Brindley PJ, Meyer CG, Velavan TP. Parasite infection, carcinogenesis and human malignancy. EBioMedicine. 2017;15:12-23. PubMed PMID: 27956028
8. Feng M, Cheng X. Parasite-associated cancers (blood flukes/liver flukes). Advances in Experimental Medicine and Biology. 2017;1018:193-205. PubMed PMID: 29052139
9. Qian M-B, Keiser J, Utzinger J, Zhou X-N. Clonorchiasis and opisthorchiasis: epidemiology, transmission, clinical features, morbidity, diagnosis, treatment, and control. Clinical Microbiology Reviews. 2024;37(1):e0000923. PubMed PMID: 38169283
10. Khuntikeo N, Andrews RH, Petney TN, Khan SA. Introduction. Recent Results in Cancer Research. 2023;219:1-5. PubMed PMID: 37660328
11. Yosry A. Schistosomiasis and neoplasia. Contributions to Microbiology. 2006;13:81-100. PubMed PMID: 16627960
12. Hamid HKS. Schistosoma japonicum-associated colorectal cancer: a review. The American Journal of Tropical Medicine and Hygiene. 2019;100(3):501-505. PubMed PMID: 30560774
13. Jain S. Can Schistosoma japonicum infection cause liver cancer? Journal of Helminthology. 2025;99:e11. PubMed PMID: 39924660
14. von Bülow V, Lichtenberger J, Grevelding CG, Falcone FH, Roeb E, Roderfeld M. Does Schistosoma mansoni facilitate carcinogenesis? Cells. 2021;10(8):1982. PubMed PMID: 34440754
15. Fedorova OS, Kovshirina YV, Kovshirina AE, Fedotova MM, Deev IA, Petrovskiy FI, Filimonov AV, Dmitrieva AI, Kudyakov LA, Saltykova IV, Odermatt P, Ogorodova LM. Opisthorchis felineus infection and cholangiocarcinoma in the Russian Federation: a review of medical statistics. Parasitology International. 2017;66(4):365-371. PubMed PMID: 27474689
16. Pakharukova MY, Mordvinov VA. Similarities and differences among the Opisthorchiidae liver flukes: insights from Opisthorchis felineus. Parasitology. 2022;149(10):1306-1318. PubMed PMID: 35570685

PubMed Topic Searches

1. Helminth infection and carcinogenesis
2. Opisthorchis viverrini and cholangiocarcinoma
3. Schistosoma haematobium and bladder cancer
4. Global burden of infection-attributable cancer

Other Sources

1. International Agency for Research on Cancer. List of Classifications, Agents Classified by the IARC Monographs, Volumes 1–142 (last updated 23 July 2026). monographs.iarc.who.int
2. International Agency for Research on Cancer. Agents Classified by the IARC Monographs (group counts and notes). monographs.iarc.who.int
3. International Agency for Research on Cancer. List of classifications by cancer sites with sufficient or limited evidence in humans. monographs.iarc.who.int

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