Parasites are already known to cause human cancer. That is not a hypothesis. The International Agency for Research on Cancer has concluded that infection with three parasitic worms — Schistosoma haematobium, Opisthorchis viverrini and Clonorchis sinensis — can cause malignancies of the bladder or bile ducts. Another worm, Schistosoma japonicum, is also believed to cause certain types of cancer.
The scale of human exposure should make the cancer question impossible to treat as a niche concern. Parasitic infection is one of the most common long-term biological exposures in the world. Worms and other parasites infect hundreds of millions of people, and many can remain in the body for years or decades while continuously interacting with human tissue, the immune system and the body’s repair mechanisms. The World Health Organization describes soil-transmitted helminth infections as among the world’s most common infections and has estimated that roughly 1.5 billion people are infected with roundworms, whipworms or hookworms. WHO also estimates that more than 600 million people may carry Strongyloides stercoralis, a roundworm capable of persisting for decades and frequently escaping diagnosis.
That scale matters because cancer is often a disease of accumulated damage. Chronic inflammation, repeated tissue repair, oxidative stress, altered immune surveillance and sustained cell proliferation are all recognized pathways through which malignancy can develop. Some parasites are already known to create exactly those conditions, and three parasitic worms have crossed the evidentiary threshold to be classified as human carcinogens. Against that background, the limited amount of long-term cancer research involving other widespread parasites is striking. It does not prove that those organisms are causing large numbers of cancers, but it does mean that medicine has not adequately answered an obvious question: what happens when enormous populations of parasites and worms live for years in a body, while creating infections capable of producing the same biological stresses already implicated in cancer?
Most people with parasitic infections will not develop parasite-related cancer, and there is no evidence that common worm infections explain cancer on a broad scale. But the combination of enormous exposure, long duration and biologically plausible mechanisms should have generated a much larger body of prospective research than exists today. Instead, outside a small number of well-established examples, the evidence remains fragmented, retrospective and often too weak to resolve cause and effect. That is not proof of a hidden epidemic. It is evidence of a substantial research gap — one large enough that the true contribution of chronic parasitic infection to cancer remains far less certain than it should be.
Worms and Parasites Are Far More Common Than Many People Realize
Humans acquire parasites in remarkably ordinary ways. Eggs from intestinal worms can contaminate soil where sanitation is inadequate and then reach people through unwashed vegetables, contaminated water or dirty hands. Hookworm and Strongyloides larvae can penetrate skin after contact with contaminated soil. Schistosome larvae penetrate skin during contact with infested freshwater. Other parasites are acquired from undercooked meat, raw freshwater fish, crabs, mollusks or aquatic plants. Protozoa can spread through contaminated food and water, while others are transmitted by mosquitoes, sand flies and other insects.
In wealthy countries with modern sanitation, many of the intestinal worms common in tropical regions are far less prevalent. But they have not disappeared from human life. International travel, migration, imported foods, raw or undercooked seafood and meat, animal exposure and localized transmission continue to expose people to a wide range of parasites. Some infections are obvious; others are remarkably quiet.
Strongyloides is particularly instructive. Its larvae penetrate the skin, migrate through the body and eventually settle in the small intestine. Unlike most common intestinal worms, however, Strongyloides can reproduce within its human host. The resulting cycle of autoinfection can allow the parasite to persist for decades without a new environmental exposure. CDC notes that the infection is frequently associated with warm, wet climates, agricultural exposure and inadequate sanitation, while WHO says the organism is often missed because it requires different diagnostic methods from other common intestinal worms.
Tapeworms present yet another model. The dwarf tapeworm, Hymenolepis nana, can complete its entire life cycle inside the human intestine and can persist through autoinfection. A landmark report in The New England Journal of Medicineestimated that as many as 75 million people could be carriers worldwide and noted prevalence as high as 25 percent among children in some locations.
The significance of these numbers is not that ordinary worm infection should be viewed as a cancer diagnosis. It should not. The significance is that chronic infection is common enough, varied enough and persistent enough to make its long-term interaction with cancer biology a legitimate area for serious investigation.
Beyond the Three Proven Cancer-Causing Worms
A systematic review published in the International Journal of Cancer tried to answer exactly that question by examining parasites other than the three recognized carcinogenic worms. Researchers searched 1,266 studies but found only 19 that met the criteria for detailed evaluation. Those studies included evidence involving Echinococcus, Strongyloides, Fasciola, Trichuris and several other parasites, as well as protozoa. The authors did not conclude that these organisms cause cancer. They concluded that the available evidence was too limited to establish cause and effect and that more investigation was warranted.
The mechanisms identified in that review are notable because they closely resemble mainstream cancer biology: chronic inflammation, sustained cell proliferation, disruption of glucose metabolism, oxidative stress, genomic instability, interference with tumor-suppressor proteins, stimulation of new blood vessels, resistance to cell death and changes associated with invasion and metastasis.
A broader 2024 review reached a similar conclusion. Beyond the established flukes, it examined possible relationships involving Strongyloides, malaria, Leishmania and trypanosomes and described several plausible pathways through which parasitic infection could influence carcinogenesis. The evidence was uneven and often preliminary, but the subject remains active precisely because the underlying biology is plausible.
One of the more obvious unresolved cases is Schistosoma japonicum. It is closely related to S. haematobium, the proven bladder carcinogen, but primarily affects the intestines and liver. IARC has classified S. japonicum as possibly carcinogenic to humans since 1994. Studies have reported elevated liver-cancer, colorectal and hepatic cancers correlated to S. japonicum worms living in the human body.
That is an important example of the problem. Medicine already knows that one member of the Schistosoma genus causes cancer. A closely related species remains under suspicion more than three decades after being placed in IARC’s “possibly carcinogenic” category. That does not prove that the second worm causes cancer. It does show how slowly the evidentiary process can move when long-term population studies are sparse.
The Drugs Used to Treat Worms and Parasites Are Often Simple and Well Established
Many common parasitic worm infections are treatable with medications that have been used for decades and, when prescribed appropriately, generally have well-characterized safety records. Ivermectin is the first-line treatment for Strongyloides infection and is typically given for only one or two days. Albendazole and mebendazole are widely used against intestinal roundworms, hookworms and whipworms, while praziquantel is the standard treatment for schistosomiasis and several fluke infections. In many cases, treatment lasts only a few days. CDC describes treatment for common soil-transmitted worms as highly effective, and the same drugs have been administered on a massive scale through public-health deworming programs around the world.
WHO and national health agencies have used ivermectin, albendazole, mebendazole and praziquantel in programs treating millions of people, and serious adverse reactions are extremely uncommon when the correct drug and dose are used for the correct infection.
The important distinction for the cancer question is that treating a carcinogenic parasite may remove the continuing source of inflammation and tissue injury, but it should not be confused with treating an established cancer. Once malignant cells have developed, killing the parasite does not necessarily reverse the mutations or cellular changes already present. The strongest medical argument for identifying and treating carcinogenic parasites is therefore prevention: stopping years of chronic infection before enough damage accumulates to help produce a malignancy.
Interest in ivermectin has moved beyond parasite treatment and into experimental cancer therapy. Some patients are already taking ivermectin off-label, often alongside standard treatments, after laboratory studies found that the drug can inhibit cancer-cell growth, trigger apoptosis and interfere with pathways such as Wnt/β-catenin and Akt/mTOR. A 2025 review concluded that ivermectin has shown anticancer activity across multiple preclinical models but emphasized that large randomized human trials have not yet confirmed a therapeutic benefit – because those studies have not been done at a scale that would prove it. A 2026 prospective observational study of 197 cancer patients taking ivermectin together with mebendazole reported that some participants described tumor regression, stable disease or no evidence of disease after six months, but the study relied heavily on patient-reported outcomes, many participants were simultaneously receiving chemotherapy, radiation, surgery or other treatments, so it has been difficult to conclude the real results. Although there is overwhelming evidence to support the theory that parasites and worms are causing cancer, the research has simply not been done by the U.S. Health and Human Services, or its agencies.
These anti-parasitic drugs have been dispensed to millions of people around the world for many decades and they are considered extremely safe for human ingestion. Ivermectin has a long history of human use as an antiparasitic medicine and a comparatively well-characterized safety profile at approved doses, while laboratory studies suggest effects on cancer-cell proliferation, cancer stem cells, mitochondrial function and several signaling pathways involved in tumor growth. But reports of tumors shrinking after ivermectin cannot yet establish if ivermectin caused the improvement, especially when patients are receiving other cancer therapies at the same time, since independent analysis has not been performed by the established medical community. Again, we do not know because the research has, strangely, not been done.
Parasites May Not Need to Cause Cancer by Themselves
The most interesting research may involve parasites that act not as single causes but as biological accomplices.
Strongyloides stercoralis has repeatedly been studied in people infected with HTLV-1, the virus that causes adult T-cell leukemia/lymphoma. A 2023 review of the disease noted evidence that Strongyloides coinfection promotes development of the leukemia/lymphoma, while earlier research has found higher HTLV-1 proviral loads in coinfected patients. Scientists are still trying to determine exactly how the two organisms interact.
The distinction between a “cause” and a “cofactor” may be important scientifically, but biologically the line can become blurry. Cancer often develops through several steps. If one infection stimulates abnormal cell proliferation, another weakens immune surveillance and a mutation then provides the final malignant change, there may be no single cause in the conventional sense.
Malaria provides a powerful example. The geographical overlap between intense Plasmodium falciparum malaria and endemic Burkitt lymphoma in Africa has been recognized for decades. Recent research continues to examine how malaria and Epstein-Barr virus interact. A 2026 review describes a model in which repeated malaria stimulates B cells and increases the opportunity for the characteristic MYC genetic rearrangement seen in Burkitt lymphoma, while EBV helps abnormal cells survive. The exact pathway remains under investigation, but the parasite appears to be considerably more than an innocent bystander.
This is precisely the sort of relationship that would be easy to miss if researchers insist on finding one organism directly responsible for one tumor. A parasite may instead create the conditions under which another infection becomes oncogenic.
Toxoplasma Presents an Even More Complicated Case
Toxoplasma gondii is among the most intriguing organisms in this field because it can establish lifelong latent infection in neural and other tissues. Epidemiological studies have raised questions about a connection to brain tumors. A systematic review and meta-analysis involving 2,323 patients with brain tumors and more than 5,000 controls found evidence of previous Toxoplasma infection in 24.2 percent of tumor patients compared with 12.9 percent of controls, producing an overall association approaching a doubling of the odds. The authors were careful to call the evidence suggestive and said properly designed prospective studies were still needed.
Subsequent research has made the story more complicated rather than less. A 2026 review found epidemiological studies pointing in both directions and noted that cancer-induced immunosuppression could reactivate latent Toxoplasma, creating the appearance that the parasite preceded a tumor when the cancer actually preceded the detectable infection.
Laboratory research is equally contradictory. A recent systematic review examined 54 experimental studies of Toxoplasmaand cancer. Forty-six reported antitumor effects, while a small number found tumor-promoting activity or signaling effects associated with chronic infection. Researchers are now investigating parasite-derived molecules not only as possible cancer promoters but also as potential cancer immunotherapies.
The Tapeworm Case That Changed the Rules
Perhaps no case illustrates the strange boundary between parasites and cancer better than what CDC researchers discovered in a Colombian man with HIV.
Doctors found malignant-looking tumors in his lymph nodes and lungs, but the cells were unusually small. Molecular testing eventually revealed that the malignant cells were not human at all. They contained DNA from Hymenolepis nana, the dwarf tapeworm.
The parasite itself had developed genetically altered, proliferating cells. Those malignant tapeworm cells then invaded human tissues and formed tumor-like masses. Researchers described it in The New England Journal of Medicine as a new disease mechanism linking parasitic infection and cancer.
This was not an example of a worm causing human cells to become cancerous, and it appears to be extraordinarily rare. But it demonstrated something almost unimaginable before the case was identified: cancer originating in a parasite can, under unusual circumstances, grow inside a human body.
Why the Evidence Is Still So Thin
For all of these leads, the most striking finding is how little decisive research exists. The systematic review that searched more than 1,200 papers and found only 19 sufficiently relevant studies on non-established parasites is telling.
The time lag between first infection and cancer compounds the problem. Cancer may appear decades after the original infection. By then, the worm may be dead, the infection treated or the evidence of exposure difficult to reconstruct. Even when the organism persists, doctors ordinarily investigate a cancer patient for causes that are already known to matter for that tumor; they do not routinely conduct exhaustive searches for every parasite the patient may have carried during the previous 30 years.
The distinction between association and causation is also difficult. Cancer can suppress immunity and allow dormant infections to reactivate. Chemotherapy can do the same. A parasite discovered after cancer develops may therefore be a consequence of the malignancy rather than its cause. The only convincing way around that problem is through prospective studies that identify infection first and then follow large numbers of people for years or decades.
Very few such studies exist.
A Field Waiting for Better Tools
The scientific opportunity today is considerably better than it was even 20 years ago. Modern tumor sequencing can identify microbial genetic material buried within tissue samples. Metagenomics can search broadly for organisms rather than testing only for pathogens that investigators already suspect. Single-cell sequencing can reveal how infection changes particular immune or epithelial cells. Proteomics can detect parasite-derived molecules, while immune profiling can sometimes reconstruct evidence of previous exposure after the organism itself has disappeared.
These technologies will not automatically prove that parasites cause additional cancers. They may instead rule out many proposed relationships. That would be useful, too.
But the work needs to be done.
That is especially true because the fundamental premise is already settled. Parasites are capable of causing human cancer. They can produce chronic inflammation, manipulate immunity and alter cell proliferation. One remarkable tapeworm has even been documented developing malignant cells of its own that invaded human tissue.
For some cancers, the initiating event may have occurred years before a tumor was visible — during an infection that produced chronic inflammation, repeatedly forced damaged cells to repair themselves or quietly altered immune surveillance. By the time cancer was diagnosed, the organism may have disappeared and the evidence linking the two events with it.
Modern oncology can now read a tumor’s genome in extraordinary detail. What it cannot always read is the patient’s biological history.
That is where the evidence leaves us. Medicine already knows that some parasites cause cancer and that hundreds of millions of people live with chronic perasitic infections that can persist for years or decades before being treated, if at all.
The real question is no longer whether parasites can cause cancer. It is how many other links have gone undetected because the studies have been too limited, too short or never done at all.
We know the connection is real. What we still do not know is how far it reaches.
SOURCES:
International Agency for Research on Cancer — Schistosomes, Liver Flukes and Helicobacter pylori
This is the foundational IARC monograph establishing the carcinogenic relationship for Schistosoma haematobium and Opisthorchis viverrini, and discussing S. japonicum and Clonorchis sinensis. IARC Publications
IARC Monograph: Schistosomes and Liver Flukes
IARC — 2025–2029 Monographs Priority Report: Schistosoma japonicum
Useful for showing that S. japonicum remains officially classified as possibly carcinogenic and is still considered important enough for renewed evaluation. IARC Monographs
IARC Priority Review of Schistosoma japonicum
World Health Organization — Soil-Transmitted Helminth Infections
Supports the prevalence section: WHO estimates about 1.5 billion people carry major soil-transmitted helminths and more than 600 million may carry Strongyloides stercoralis. World Health Organization
WHO: Soil-Transmitted Helminth Infections
CDC — Strongyloides
Strong source for explaining how Strongyloides is acquired, how often it produces no symptoms, and how autoinfection can allow it to persist in the body. CDC
CDC: Strongyloides Overview
International Journal of Cancer — Carcinogenesis Associated With Parasites Other Than the Established Flukes
Probably the most important paper for the broader thesis. The systematic review examined possible cancer relationships involving Strongyloides, Echinococcus, Fasciola, Trichuris and other parasites and emphasized how limited the evidence base remains.
PubMed: Parasites and Carcinogenesis Systematic Review
2026 Review — Parasitic Infections and Carcinogenesis: Molecular Mechanisms, Immune Modulation, and Emerging Therapeutic Strategies
An excellent recent review tying parasitic infection to chronic inflammation, genomic instability, altered signaling and parasite-derived exosomes. It directly states that the oncogenic mechanisms remain insufficiently understood. PubMed
PubMed: Parasitic Infections and Carcinogenesis
Toxoplasma gondii and Brain Tumors — Systematic Review and Meta-Analysis
Useful for the section on possible links beyond worms. It found a statistical association between prior Toxoplasmaexposure and brain tumors while stopping short of proving causation.
PubMed: Toxoplasma and Brain Tumors Meta-Analysis
Malaria, Epstein-Barr Virus and Endemic Burkitt Lymphoma — 2026 Review
A current review of how chronic Plasmodium falciparum exposure may alter immunity, promote EBV persistence and increase genomic instability involved in Burkitt lymphoma. PubMed
PubMed: Malaria, EBV and Burkitt Lymphoma
Endemic Burkitt Lymphoma and Plasmodium falciparum
Useful epidemiological support: the review notes that Burkitt lymphoma prevalence is substantially higher in areas of stable P. falciparum transmission and discusses the malaria-EBV interaction. PubMed
PubMed: Plasmodium falciparum and Burkitt Lymphoma
Strongyloides and HTLV-1 / Adult T-Cell Leukemia-Lymphoma
Good supporting source for the theory that parasites may function as cofactors rather than sole causes of cancer.
PubMed: Strongyloides, HTLV-1 and Adult T-Cell Leukemia-Lymphoma
New England Journal of Medicine — Malignant Transformation of Hymenolepis nana in a Human Host
The extraordinary documented case in which malignant cells arising from a dwarf tapeworm invaded human tissues. It is not conventional parasite-caused human cancer, but it is one of the most striking papers in the entire field. New England Journal of Medicine
NEJM: Malignant Transformation of Hymenolepis nana
CDC — Liver Flukes and Bile-Duct Cancer
A clear public-health source confirming that long-standing Clonorchis and Opisthorchis infections are associated with cholangiocarcinoma and explaining how the infections are acquired. CDC
CDC: Clonorchis Clinical Overview
