Are Parasites Inside Every Tumor?

Table of Contents

1. What the Presentation Said
2. Where the Video Appears to Come From
3. What Fact-Checkers Have Written
4. What Looks Like a Parasite Under the Microscope
5. Bacteria Really Do Live Inside Many Tumours
6. Fungi, Including Candida, Inside Tumours
7. Fusobacterium and Colorectal Cancer
8. A Retracted Study and the Contamination Problem
9. Microbes That Are Proven Causes of Cancer
10. How the Claim Compares With the Research
11. What Is Not Known
12. Connections
13. Key Research Papers
14. Featured Videos

What the Presentation Said

Early in the talk the speaker said that, in a cancer situation, parasites are part of the tumour microenvironment — the mix of cells, blood vessels and other material that surrounds and feeds a tumour. Later the speaker played a video, with its sound not working, and described it to the audience.

As the speaker described it, the video showed German oncologists who had cut open tumours and shown that cancer is caused by parasites or cysts. The narration, partly audible in the transcript, pointed to microscope images of cells from a lung carcinoma, with red blood cells visible in one corner, and tumour cells from a bladder carcinoma, with vacuoles and string-shaped structures. The video's conclusion, as repeated by the speaker, was that there is a certain number of microbes in all tumours, and that there is no tumour tissue without these micro-parasites.

The speaker added a second idea to support this: that parasites drain their host of nutrients until the host dies, and that Western medicine calls this cancer. The speaker then linked the video to the theory of a doctor in Italy who holds that all cancers contain Candida, a yeast-like fungus, described in the talk as one kind of micro-parasite.

The speaker did not name the oncologists, the video, a journal or a study, and no publication date was given.

Where the Video Appears to Come From

No published study matching the video was found. Searches of PubMed for tumour-tissue microscopy showing parasites in all cancers returned nothing of the kind, and no peer-reviewed paper by a group of German oncologists making that claim turned up in web searches either.

The description resembles a line of work that circulated in Germany in the second half of the twentieth century, in which individual practitioners described single-celled “cancer protozoa” in blood and tumour samples viewed under a light microscope, and filmed what they saw. That work was spread through lectures, films and books rather than peer-reviewed journals, and no published study has confirmed it.

Which film was shown in the 2026 presentation could not be determined, because the speaker did not identify it. No peer-reviewed study matching its description — micro-parasites in lung and bladder carcinoma cells, and no tumour tissue free of them — was found.

What Fact-Checkers Have Written

No fact-check of this exact video was found. Related claims have been reviewed:

• Science Feedback, reviewing viral "copypasta" posts about cancer that circulated in January 2025, rated the claim that "cancer starts with a parasite" as incorrect. Its review states that cancer results from mutations that make cells grow without control, that infection with certain microorganisms, including specific parasites, is one of several factors that can raise the risk of some cancers, and that cancer is not caused by parasites alone.
• Correctiv, a German fact-checking newsroom, examined a different microscope video in December 2021 (one claiming to show worms in vaccinated people's blood). The specialists it quoted, from university hospitals in Leipzig and Munich, said the supposed organisms were most likely contamination, foreign bodies, or structures that form as liquids dry on a slide, and that dark-field microscopy of a drop of blood is not an established diagnostic method. That review is about a different video, but it describes the same kind of evidence.

Both outlets are listed under Other Sources below.

What Looks Like a Parasite Under the Microscope

The two structures the video pointed to — red blood cells in a lung-cancer sample and vacuoles in bladder-cancer cells — are ordinary parts of tissue.

• Red blood cells are present in almost every tissue sample, because tumours have a blood supply and cutting tissue releases blood. They are small, round and have no nucleus.
• Vacuoles are fluid-filled spaces inside cells. Cancer cells and normal cells both form them, for example when they take in fluid or break down their own material.
• String-shaped structures in a tissue sample can be strands of mucus, protein fibres such as fibrin, or fibres from paper or cotton.

Parasitologists' own list of look-alikes

Mistaking ordinary objects for parasites is a known problem inside parasitology itself, where the look-alikes are called pseudoparasites. A 2025 commentary by two medical parasitologists at a teaching hospital in Chandigarh, India, lists the common ones (Singh and Khurana, 2025):

• Cells from the patient, such as lining (epithelial) cells, white blood cells and mucus threads, which can be mistaken for amoebae.
• Yeast cells and fungal spores, mistaken for protozoan cysts such as Giardia, or for worm eggs.
• Pollen grains mistaken for roundworm (Ascaris) eggs, and plant or cotton fibres mistaken for Strongyloides larvae.
• Air bubbles and stain deposits that take up dye and resemble Cryptosporidium.
• In blood smears, degenerating platelets that can look like trypanosomes or malaria parasites, and small pieces of nuclear material inside red cells (Howell–Jolly bodies) that can be misread as Plasmodium.

The authors write that inexperience, delays in processing, poor fixation and contamination during slide preparation all add to the problem, and that doubtful findings are normally confirmed with antigen, antibody or PCR (DNA) tests. A 2020 pathology study made the same point for food: partly digested fruit and vegetables in gut biopsies can mimic parasites, drug resins and other significant findings under the microscope (Razzano and Gonzalez, 2020).

These papers do not examine the video. They show why a microscope image alone, without staining, controls or a DNA test, is not accepted as proof that a structure is a living organism.

Bacteria Really Do Live Inside Many Tumours

The idea that microbes sit inside tumours is not fringe in itself. What modern research has found is bacteria and fungi, usually in very small amounts — not worms or protozoa in every tumour.

The 2020 Weizmann Institute study

A team led from the Weizmann Institute in Israel studied 1,526 tumours and the normal tissue next to them, across seven cancer types: breast, lung, ovary, pancreas, melanoma, bone and brain (Nejman and colleagues, 2020). The authors note that bacteria were first reported in human tumours more than 100 years ago, but that measuring them has been hard because there is so little microbial material (low "biomass"). They reported:

• Each tumour type had its own distinct mix of bacteria; breast cancer had a particularly rich and varied one.
• The bacteria were mostly inside cells — both cancer cells and immune cells.
• The bacteria present were linked with tumour type and subtype, patients' smoking history and response to immunotherapy. These are associations, not proof that the bacteria caused the cancers.

Pancreatic cancer and survival

A 2019 study of people whose pancreatic cancer had been surgically removed compared short-term and long-term survivors (Riquelme and colleagues, 2019). Long-term survivors had a more diverse bacterial community in their tumours, and a particular bacterial signature predicted long survival in both the discovery and the validation groups. When the researchers transplanted stool from these patients into mice, the tumour bacteria, tumour growth and immune response in the mice changed. That transplant work was done in mice.

Note what these studies found: bacteria, at low levels, in some tumours, with patterns that differ between cancers. Neither describes parasites, and neither reports that every tumour contains microbes.

Fungi, Including Candida, Inside Tumours

Because the presentation linked tumours to Candida, the fungal studies matter here. Two large studies were published side by side in the journal Cell in 2022.

Fungi across 35 cancer types

One study examined 17,401 tissue, blood and plasma samples from patients with 35 cancer types in four independent groups (Narunsky-Haziza and colleagues, 2022). The researchers reported fungal DNA and fungal cells at low abundance across many major cancers, with different fungal communities in different cancers even after allowing for background contamination. Staining of tissue samples supported that the fungi were inside the tumours, often close to cancer cells and macrophages (a type of immune cell). The authors suggested that fungal patterns might one day help with diagnosis or outlook. Several authors declared patents and company interests in microbial cancer diagnostics.

Candida in gut tumours

The second study found tumour-associated fungi at up to 1 fungal cell per 10,000 tumour cells (Dohlman and colleagues, 2022). Its main findings:

• In stomach cancers, high levels of Candida went with switched-on inflammatory immune pathways.
• In colon cancers, Candida predicted spread to other organs (metastatic disease).
• Across several digestive-tract sites, Candida species were more common in tumour samples, and tumour-associated Candida DNA was linked to shorter survival.
• In lung cancer, a different fungus, Blastomyces, was associated with tumour tissue.

The authors wrote that these data "implicate" fungi in digestive cancers and that fungal DNA might serve as a biomarker. They did not show that Candida causes cancer, and Candida was not found in all cancers.

Long-term Candida infection and cancer of the gullet

A separate line of research concerns people with chronic mucocutaneous candidiasis, a rare immune condition in which Candida infections of the mouth, skin and nails keep returning from childhood. A 2008 report described three such patients — two men and the mother of one of them — who developed squamous-cell cancer of the oesophagus (gullet); all three died of it (Rosa and colleagues, 2008). The authors noted earlier reports of the same link in a related syndrome. This is a case series in a rare condition, not evidence about cancer in general.

The theory that all cancer is Candida

No clinical trial, cohort study or laboratory study testing the theory that every cancer is a Candida infection was found in PubMed. The published fungal studies above report fungi in some tumours, at low levels and with different species in different cancers, which is a different finding from the theory as stated in the talk.

Fusobacterium and Colorectal Cancer

The best-studied example of a microbe living inside a tumour is Fusobacterium nucleatum, a bacterium normally found in the mouth that is rare in the lower gut of healthy people.

• Discovery, 2012. Two research groups reported in the same issue of Genome Research that Fusobacterium DNA was over-represented in colorectal tumours compared with normal bowel tissue. One confirmed it in 95 tumour/normal pairs and saw the bacteria inside tumours with a fluorescent stain (Kostic and colleagues, 2012). The other confirmed it in 99 people, grew a live, invasive Fusobacterium from a frozen tumour, and found a link with spread to lymph nodes (Castellarin and colleagues, 2012).
• It travels with the cancer. In a 2017 study, Fusobacterium and its companion bacteria were still present in distant metastases of the same patients' colorectal cancers, mainly next to cancer cells. In mice carrying human colorectal tumours, the antibiotic metronidazole reduced the bacterial load and slowed tumour growth (Bullman and colleagues, 2017). That antibiotic result was in mice; the authors called for further study, not treatment.
• Chemotherapy response. Fusobacterium was abundant in tumours of patients whose colorectal cancer came back after chemotherapy, and laboratory work showed it can switch on a cell-survival process called autophagy that blunts chemotherapy (Yu and colleagues, 2017).
• One strain dominates. A 2024 study sequencing 135 strains found that one specific group (clade) of the bacterium, not the whole species, is the one enriched in colorectal tumours, in 116 patients' tumour samples and in stool from 627 people with colorectal cancer compared with 619 without it (Zepeda-Rivera and colleagues, 2024).

Fusobacterium is a bacterium, not a parasite, and it is found in a portion of colorectal tumours, not all of them. Whether it starts colorectal cancer or mainly thrives in, and speeds up, a tumour that already exists is still debated.

A Retracted Study and the Contamination Problem

The tumour-microbiome field has had a serious correction that bears directly on claims that microbes are found in all cancers.

In 2020 the journal Nature published a widely cited study that used DNA reads from a large public cancer-genome database to claim that each cancer type has its own microbial signature, detectable even in blood. In 2023 a team of genome scientists re-analysed the same data and reported "major data analysis errors": most of the microbes originally reported were not present in the samples at all, and the original report and more than a dozen follow-up studies were, in their words, likely to be invalid (Gihawi and colleagues, 2023). Nature retracted the 2020 paper in July 2024; the retraction notice is indexed in PubMed (Poore and colleagues, 2024). That paper is not cited on this page as evidence of anything.

In 2025 some of the same researchers analysed whole-genome sequencing from 8,908 patients with 22 cancer types in a large UK dataset (Gihawi and colleagues, 2025). They found that only colorectal tumours had clearly distinct microbial communities, confirmed in two other datasets, and they wrote that their results "contradict previous claims that each cancer type has a distinct microbiological signature". They did detect known cancer-causing viruses (human papillomavirus in oral cancers, and the rare HTLV-1 virus) and some links between anaerobic bacteria and survival in certain sarcomas.

The lesson for the "no tumour without microbes" claim is that when there is very little microbial material, contamination from reagents, the air, the skin or the laboratory can look like a real signal. That is true of DNA sequencing, and it is true of a microscope slide.

Microbes That Are Proven Causes of Cancer

None of this means microbes are irrelevant to cancer. A few are well-established causes, and the research on them looks very different from a microscope video:

• Helicobacter pylori, a stomach bacterium, is a recognised cause of stomach cancer. A Cochrane review of seven randomised trials with 8,323 healthy infected adults found that eradicating the bacterium roughly halved later stomach cancer (risk ratio 0.54) and reduced deaths from stomach cancer (risk ratio 0.61). Six of the seven trials were in Asian populations, and the authors cautioned that the results may not carry over to other populations (Ford and colleagues, 2020).
• Viruses such as human papillomavirus and HTLV-1, detected in the 2025 genome study above.
• Three parasites — the blood fluke Schistosoma haematobium and the liver flukes Opisthorchis viverrini and Clonorchis sinensis — are classed by the International Agency for Research on Cancer as causes of cancer in people, for specific cancers in the regions where they are common. These are covered on the sibling pages on bladder cancer and bile duct cancer.

One of the 2012 Fusobacterium papers and the 2025 genome study both cite estimates that known infections cause about 15% of cancers worldwide (Castellarin and colleagues, 2012; Gihawi and colleagues, 2025). By the same estimate, the large majority of cancers have no known infectious cause.

How the Claim Compares With the Research

What the research supports

• Microbes can be part of the tumour microenvironment. Published studies report bacteria inside cancer and immune cells in several tumour types, and fungi, including Candida, in some digestive-tract tumours.
• A small number of true parasites are accepted causes of specific cancers.

What it supports only in part

• Candida and cancer: published studies link tumour Candida to inflammation, spread and shorter survival in some digestive cancers, and long-term Candida infection in a rare immune disorder to cancer of the gullet. They do not show that all cancers contain Candida or that Candida causes cancer in general.

What it contradicts

• "No tumour tissue without parasites": the large sequencing studies found bacteria and fungi, not parasites, at low levels, with patterns that vary by cancer type. The largest recent genome study found a clearly distinct microbial community only in colorectal cancer.
• Cancer as parasites draining the host: cancer is the uncontrolled growth of the body's own cells after genetic changes, which is how Science Feedback summarised it in its review of the "cancer starts with a parasite" claim. Tumour tissue is made of human cells.
• The structures named in the video — red blood cells and vacuoles — are normal parts of tissue and cells.

What has no research at all

• No peer-reviewed study by German oncologists showing parasites in all tumour tissue was found.
• No clinical trial or laboratory study testing the theory that every cancer is a Candida infection was found in PubMed.

Decisions about cancer treatment and antiparasitic medicines belong with a clinician.

What Is Not Known

• Cause or passenger? For most tumour bacteria and fungi, researchers do not know whether they help start cancer, speed it up, or simply settle in tissue that is already abnormal, with leaky blood vessels and weakened immune defences.
• How much is real. After the 2024 retraction, which tumour-microbe findings survive careful contamination control is still being worked out, especially for low-biomass tumours such as breast, brain and bone.
• Treatment. The metronidazole result for Fusobacterium-carrying tumours was in mice. No clinical trial was found showing that antibiotics, antifungals or antiparasitic drugs improve survival by clearing microbes from tumours.
• The video itself. Without the original source, the method, samples and controls behind the images in the presentation cannot be checked.

Connections

• Cancer and Parasites
• Fibiger and the Parasite That Won a Nobel Prize
• Parasites Classified as Cancer-Causing Agents
• How Parasites Can Drive Cancer: The Mechanisms
• Stool Tests for Parasites and Their Limits
• Cancer
• Colorectal Cancer
• Pancreatic Cancer
• Stomach Cancer
• Esophageal Cancer
• H. pylori and Gastric Cancer Risk
• The Oral Microbiome
• Fungal Infections
• Parasites
• Bacteria

Key Research Papers

1. Nejman D, Livyatan I, Fuks G, et al. The human tumor microbiome is composed of tumor type-specific intracellular bacteria. Science. 2020;368(6494):973-980. PubMed PMID: 32467386
2. Riquelme E, Zhang Y, Zhang L, et al. Tumor Microbiome Diversity and Composition Influence Pancreatic Cancer Outcomes. Cell. 2019;178(4):795-806.e12. PubMed PMID: 31398337
3. Narunsky-Haziza L, Sepich-Poore GD, Livyatan I, et al. Pan-cancer analyses reveal cancer-type-specific fungal ecologies and bacteriome interactions. Cell. 2022;185(20):3789-3806.e17. PubMed PMID: 36179670
4. Dohlman AB, Klug J, Mesko M, et al. A pan-cancer mycobiome analysis reveals fungal involvement in gastrointestinal and lung tumors. Cell. 2022;185(20):3807-3822.e12. PubMed PMID: 36179671
5. Rosa DD, Pasqualotto AC, Denning DW. Chronic mucocutaneous candidiasis and oesophageal cancer. Medical Mycology. 2008;46(1):85-91. PubMed PMID: 17852718
6. Kostic AD, Gevers D, Pedamallu CS, et al. Genomic analysis identifies association of Fusobacterium with colorectal carcinoma. Genome Research. 2012;22(2):292-298. PubMed PMID: 22009990
7. Castellarin M, Warren RL, Freeman JD, et al. Fusobacterium nucleatum infection is prevalent in human colorectal carcinoma. Genome Research. 2012;22(2):299-306. PubMed PMID: 22009989
8. Bullman S, Pedamallu CS, Sicinska E, et al. Analysis of Fusobacterium persistence and antibiotic response in colorectal cancer. Science. 2017;358(6369):1443-1448. PubMed PMID: 29170280
9. Yu T, Guo F, Yu Y, et al. Fusobacterium nucleatum Promotes Chemoresistance to Colorectal Cancer by Modulating Autophagy. Cell. 2017;170(3):548-563.e16. PubMed PMID: 28753429
10. Zepeda-Rivera M, Minot SS, Bouzek H, et al. A distinct Fusobacterium nucleatum clade dominates the colorectal cancer niche. Nature. 2024;628(8007):424-432. PubMed PMID: 38509359
11. Gihawi A, Ge Y, Lu J, et al. Major data analysis errors invalidate cancer microbiome findings. mBio. 2023;14(5):e0160723. PubMed PMID: 37811944
12. Poore GD, Kopylova E, Zhu Q, et al. Retraction Note: Microbiome analyses of blood and tissues suggest cancer diagnostic approach. Nature. 2024;631(8021):694. (The retraction notice, cited only to document the retraction.) PubMed PMID: 38926587
13. Gihawi A, Wood HM, Clark J, et al. The landscape of microbial associations in human cancer. Science Translational Medicine. 2025;17(814):eads6166. PubMed PMID: 40901921
14. Ford AC, Yuan Y, Forman D, et al. Helicobacter pylori eradication for the prevention of gastric neoplasia. Cochrane Database of Systematic Reviews. 2020;7(7):CD005583. PubMed PMID: 32628791
15. Singh S, Khurana S. Not everything that wiggles is a worm: Pseudoparasites in parasitology. Tropical Parasitology. 2025;15(2):138-140. PubMed PMID: 41244082
16. Razzano D, Gonzalez RS. Disease, drugs, or dinner? Food histology can mimic drugs and parasites in the gastrointestinal tract. Virchows Archiv. 2020;477(4):593-595. PubMed PMID: 32170449

PubMed Topic Searches

1. Intratumoral microbiome
2. Tumour mycobiome and Candida
3. Fusobacterium nucleatum in colorectal cancer tissue
4. Contamination in low-biomass cancer microbiome studies
5. Pseudoparasites

Other Sources

1. Science Feedback, review of viral posts claiming cancer starts with a parasite (January 2025 posts). science.feedback.org
2. Correctiv, fact-check of a microscope video of blood samples, 20 December 2021 (German). correctiv.org

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