Fibiger and the Parasite That Won a Nobel Prize

In 1927 the Danish pathologist Johannes Fibiger received the Nobel Prize in Physiology or Medicine for 1926 for showing, as he believed, that a small worm carried by cockroaches caused stomach cancer in rats. It is the only Nobel Prize ever given for a parasite-causes-cancer discovery, and it is now widely described as a mistake. This page sets one point from a 2026 integrative-oncology presentation — that tumours are caused by parasites — beside this older, much-studied version of the same idea, using Fibiger's own Nobel lecture, the Nobel Prize archives and the historical research that re-examined his work.

What the Presentation Said
Cancer Theories Around 1900
Who Johannes Fibiger Was
The Rat, the Cockroach and the Worm
The 1926 Prize and the Nominations
Yamagiwa and Coal Tar: The Discovery That Lasted
What Fibiger Himself Said About Human Cancer
The Reappraisal: Vitamin A and Misread Lesions
What Survived: Parasites and Microbes That Do Cause Cancer
How the Claim Compares With the Research
What Is Not Known
Connections
Key Research Papers
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. The speaker did not mention Johannes Fibiger, his Nobel Prize, the worm he studied, or any other history of the parasite theory of cancer. This page is therefore not about a claim the speaker made about Fibiger. It looks at Fibiger's story as the best-documented earlier test of the speaker's general idea.

The general idea, as the speaker put it, ran like this:

While showing a video, the speaker said that German oncologists had proved that cancer is caused by parasites or cysts, and that they had cut tumours open and shown that the parasites were the tumours.
The speaker described microscope images said to come from that work — cells from a lung carcinoma with red blood cells nearby, and cells from a bladder carcinoma with vacuoles and string-shaped structures — and repeated the video's statement that there is no tumour tissue without parasites or microbes.
The speaker added that parasites drain their host of nutrients until the host dies, and that Western medicine calls this cancer.
The speaker linked this to a separate theory that all cancers involve the yeast Candida.

No study, authors, year or journal was named for the German work, so it could not be checked from the talk itself. A sister page, Are Parasites Inside Every Tumor?, looks at that claim and at the modern research on microbes inside tumours. This page tells the story of the one time the idea that a parasite causes cancer was tested in the laboratory, accepted by the leading cancer scientists of the day, and rewarded with the highest prize in medicine.

Cancer Theories Around 1900

In the late 1800s, bacteriology was the most successful science in medicine. Robert Koch, Louis Pasteur and others had shown that tuberculosis, cholera, anthrax and diphtheria each had a single microbial cause. It was natural to ask whether cancer did too.

Historians describe more than a century during which a "germ theory" of cancer was taken seriously. A 2014 historical review lists scientists who blamed cancer on cocci (round bacteria), fungi and parasites, and concludes that although those theories rested on "fantasy and misinterpretation", they helped start experimental cancer research (Tsoucalas and colleagues, 2014). A 2022 historical essay from Poland describes the same period: researchers used the methods of bacteriology, tried to "infect" animals and people with cancer, transplanted cancer cells into healthy animals and even tested disinfectants as a way to prevent cancer (Paliga, 2022).

The presentation speech given at the 1927 Nobel ceremony summed up the situation before Fibiger. Repeated attempts to cause cancer in animals with irritants such as tar or X-rays had failed. The search for a "cancer bacillus" had come to nothing. Cancer had also been blamed on worms, but no one had shown this in an experiment, and the speech notes that people who kept supporting the idea "were, moreover, frequently considered to be fantasts" (Nobel Prize ceremony speech, 1927).

The key problem was that no one could reliably make a normal animal develop cancer. Without that, causes could not be tested. Fibiger's worm appeared to solve the problem.

Who Johannes Fibiger Was

Johannes Andreas Grib Fibiger was born in Silkeborg, Denmark, on 23 April 1867 and died in Copenhagen on 30 January 1928, a few weeks after receiving his prize. According to his Nobel biography he qualified as a doctor in 1890, studied under the German bacteriologists Robert Koch and Emil von Behring, wrote his doctoral thesis on the bacteriology of diphtheria, and in 1900 became Professor of Pathological Anatomy at the University of Copenhagen (Nobel Prize biographical page). The same biography records that he had cancer of the colon at the time of his death.

Fibiger is remembered for a second, less famous reason. In the 1890s he ran a trial of serum treatment for diphtheria, and medical historians marking its centenary in 1998 described it as an early controlled clinical trial (Hróbjartsson and colleagues, 1998). The man whose cancer research is now remembered as an error was, in another field, an early user of careful comparison groups.

The Rat, the Cockroach and the Worm

Fibiger described his own experiments in his Nobel lecture, delivered in Stockholm on 12 December 1927. The figures below come from that lecture.

Three rats from Dorpat (1907)

In 1907 Fibiger found large papillomatous tumours (wart-like growths of the stomach lining) filling the stomachs of three wild rats that had come from Dorpat (now Tartu, Estonia). Inside the growths he found small roundworms (nematodes). He then examined nearly 1,200 wild and laboratory rats without finding the worms or the growths again, and attempts to pass the growths on by feeding tumour tissue to healthy rats failed.

The sugar refinery

The breakthrough came from rats living in a Copenhagen sugar refinery that was full of a tropical cockroach, Periplaneta americana (the American cockroach). The worm was found in the stomachs of 40 of 61 wild brown rats from the refinery; 18 had changes in the stomach and 9 had very large papillomatous growths. Fibiger worked out the worm's life cycle: rats pass the worm's eggs in their droppings, cockroaches eat the droppings, larvae develop in the cockroaches' muscles, and rats are infected when they eat the cockroaches.

Feeding experiments

When healthy rats were fed cockroaches from the refinery, the worm appeared in 54 of 57 rats; 37 showed thickening and wart-like change of the stomach lining and 7 had large papillomatous tumours.
In laboratory rats fed infected cockroaches, Fibiger reported invasive squamous-cell carcinoma (a cancer of flat, skin-like cells) in some animals, with spread (metastases) to the lungs in a minority. He reported finding lung metastases in 6 of 33 rats whose lungs were examined in serial sections.
By the time of the lecture he counted more than 100 stomach carcinomas and 7 tongue carcinomas in experimental rats. Of 102 laboratory rats that lived at least 45 days after infection, he reported stomach carcinoma in 54 — "rather more than half".
He reported that one tumour could be transplanted from mouse to mouse through four generations.

Fibiger named the worm Spiroptera neoplastica; with the zoologist Hjalmar Ditlevsen he later classified it as Gongylonema neoplasticum. All of this was work in rats and mice. The lesions grew in the forestomach, a part of the rat stomach lined with skin-like (squamous) cells that humans do not have.

The 1926 Prize and the Nominations

The Nobel Prize in Physiology or Medicine 1926 was awarded to Johannes Fibiger "for his discovery of the Spiroptera carcinoma". The Nobel Prize website explains that in 1926 the Nobel Committee decided that none of that year's nominations met the criteria in Alfred Nobel's will; under the statutes the prize was reserved, and Fibiger received the 1926 prize one year later, in 1927 (Nobel Prize 1926 summary page).

The ceremony speech praised the work in the strongest terms. It said that for the first time it had been possible "to change by experiment normal cells into cells having all the terrible properties of cancer", and that this showed "not that cancer is always caused by a worm, but that it can be provoked by an external stimulus" (Nobel Prize ceremony speech, 1927). Even at the moment of the award, the prize committee did not claim that worms cause cancer in general.

What the nomination archive shows

The Nobel Prize nomination archive shows that Fibiger and the Japanese pathologist Katsusaburo Yamagiwa were often considered together:

In 1926 the German pathologist Ludwig Aschoff nominated Fibiger for "work on carcinoma induced by the nematode Spiroptera" and Yamagiwa for "experimentally induced tumors", in a single nomination (nomination archive, 1926, number 11).
The archive notes on another 1926 nomination of Fibiger that "both evaluations included K. Yamagiwa" (nomination archive, 1926, number 29).
Yamagiwa was nominated again in later years — in 1928, for example, together with Fibiger, as the first to show that malignant tumours could be produced by applying tar to the tissues of certain mammals (nomination archive, 1928, number 23). He never received the prize.

A 2004 study of the Nobel archives by researchers at the Karolinska Institute examined how the committee reached its decision; its title calls the award "a wrong Nobel Prize" (Stolt and colleagues, 2004). A 2022 historical commentary on controversial Nobel choices lists the Fibiger award as an error of commission and gives the committee's likely reasoning: in an era when cancer was mysterious and usually fatal, it believed an animal model would push the field forward, and an earlier proposal that the blood fluke Schistosoma haematobium caused bladder cancer, together with a long-standing theory that infection led to cancer, may have contributed (Lichtman, 2022).

Yamagiwa and Coal Tar: The Discovery That Lasted

While Fibiger worked with worms, Katsusaburo Yamagiwa at the Imperial University of Tokyo took a different approach, based on Rudolf Virchow's idea that long-lasting irritation leads to cancer. Doctors had long known that chimney sweeps and some industrial workers developed skin cancers, and coal tar was suspected, but earlier animal experiments with tar had failed.

Yamagiwa and his assistant Koichi Ichikawa painted coal tar again and again on the inner surface of rabbits' ears. On 25 September 1915 they reported that they had produced papillomas, and their series of six reports (1915–1924) described the experimental production of skin cancers. A 2014 review that translated the essentials of those reports into English explains that they showed chronic inflammation to be an important cause of cancer, a mechanism still being studied today (Fujiki, 2014). The same review notes that the Japanese researcher Hidejiro Tsutsui showed in 1918 that coal tar painted on mouse skin also causes cancer, and that mouse skin became a standard tool in experimental carcinogenesis (the study of how cancer is caused).

In his Nobel lecture Fibiger himself credited Yamagiwa and Ichikawa, noted that he and a colleague had confirmed Tsutsui's mouse results in 1920, and remarked that tar cancer "develops quite easily among mice", while mice would contract the worm-induced cancer "only with the greatest difficulty" (Fibiger Nobel lecture, 1927). In hindsight, chemical carcinogenesis — cancer caused by chemicals — became the lasting experimental method of that era, as the review notes for mouse skin (Fujiki, 2014). The worm model was not.

What Fibiger Himself Said About Human Cancer

Fibiger's Nobel lecture is often overlooked in retellings of the story. He was much more cautious about people than his prize citation suggests. In the lecture he stated that:

Gongylonema neoplasticum "is never found in humans", so far as could be ascertained. A related worm that lives in pigs can occasionally infect the human lips and tongue, but in the reported patients there were only very slight changes in the lining and no cancer.
Helminths (parasitic worms) "which must be assumed to result in the development of cancer are quite few in number".
Even allowing that a worm might leave a tumour after starting it, "we are nevertheless not justified in ascribing to the Helminthes any extensive aetiological role in the pathology of cancer in Man". Research on his worm "does not give unqualified support for the theories that cancer is in general ascribable to macroparasites or microbes".
Present knowledge allowed parasitic worms "only a modest place among the causes of neoplasms among humans, a fact that I stressed in my first work back in 1913".
"There has been no convincing evidence offered for the microbial origin of cancer in Man, and all theories of this kind must be regarded as based upon insufficient grounds."

At the same time, Fibiger listed the parasites he considered likely causes of some human cancers: the blood fluke Schistosoma haematobium in bladder cancer, which he considered proven, and the liver flukes Opisthorchis felineus and Clonorchis sinensis and the blood fluke Schistosoma japonicum in some liver cancers. Several of those links are recognised today, as described below.

In short, the scientist who won a Nobel Prize for a parasite-cancer model did not believe that parasites cause most human cancers, and said so in his prize lecture.

The Reappraisal: Vitamin A and Misread Lesions

Within a few years of Fibiger's death, other researchers had trouble repeating his results. A 1997 French historical paper summarises the problems: questions over which Gongylonema species were involved, the rats' diet and lack of vitamin A, and "even the impossibility to reproduce his results" (Petithory and colleagues, 1997).

Vitamin A deficiency

Vitamin A keeps the lining of many organs healthy; when it is lacking, soft lining tissue can change into thick, hard, skin-like tissue. This kind of change is called metaplasia (one mature cell type replaced by another) and can look alarming under a microscope. In 1952 Claude Hitchcock and E. T. Bell published a long study of Gongylonema neoplasticum and of vitamin A deficiency in the rat forestomach, comparing their findings with Fibiger's (Hitchcock and Bell, 1952). A 1978 review by the Danish epidemiologist Johannes Clemmesen also examined the role of the worm and of vitamin A (Clemmesen, 1978).

The Nobel Prize website today summarises the outcome plainly: "it later turned out that the primary cause of the cancer was a lack of vitamin A instead, and that the worm larvae only caused damage to tissue, where the cancer could begin developing" (Nobel Prize facts page on Fibiger).

Lesions that were not cancer

The second problem was the diagnosis itself. Later pathologists concluded that most of what Fibiger called carcinoma was not true cancer. The 2022 commentary states that the lesions "were misidentified as neoplastic but were not" (Lichtman, 2022). A 2014 review mentions being told by a journal reviewer that Fibiger's cancers were later shown to be granulomas, a type of inflammatory lump (Fujiki, 2014) — a second-hand remark rather than a finding of that paper. Historians have since published reviews whose titles speak for themselves: "Johannes Fibiger and his Nobel Prize for the hypothesis that a worm causes stomach cancer" (Stolley and Lasky, 1992) and "An analysis of a wrong Nobel Prize" (Stolt and colleagues, 2004).

Why the error was easy to make

Later work pointed to a lack of vitamin A in the rats' diet as the main driver of the stomach changes, so diet, not the worm alone, shaped what Fibiger saw (Nobel Prize facts page on Fibiger).
Thick, wart-like squamous growths in a damaged forestomach look alarming under a microscope; the 2022 commentary states that Fibiger's lesions were misidentified as cancer (Lichtman, 2022).
The idea fitted the expectations of the time: infections had already been shown to cause many diseases, and a parasite-cancer link was widely expected.

What Survived: Parasites and Microbes That Do Cause Cancer

The Fibiger story is not proof that infections never cause cancer. The opposite turned out to be true for a small number of specific organisms. Petithory and colleagues ended their 1997 paper on Fibiger by noting that the cancer-causing action of some parasitic worms "such as Schistosoma is now recognized".

Blood and liver flukes. The International Agency for Research on Cancer (IARC) classifies Schistosoma haematobium, Opisthorchis viverrini and Clonorchis sinensis as Group 1 — known to cause cancer in humans. These are covered on Parasites Classified as Cancer-Causing Agents, Schistosoma and Bladder Cancer and Liver Flukes and Bile Duct Cancer.
A bacterium in the stomach. In 2005 Barry Marshall and Robin Warren won the Nobel Prize "for their discovery of the bacterium Helicobacter pylori and its role in gastritis and peptic ulcer disease" (Nobel Prize 2005 summary page). The prize citation names gastritis and ulcers; the bacterium's link to stomach cancer is covered on the Helicobacter Pylori page listed under Connections. Commentators have linked this discovery back to Fibiger's idea that a living organism could cause stomach cancer (Weisse, 1996; Modlin and colleagues, 2001).
Viruses. The 2022 commentary notes that a Nobel Prize was awarded for the discovery of human papillomavirus in 2008 (Lichtman, 2022).

What survived from Fibiger's era, then, was the idea that a few specific infections raise the risk of a few specific cancers, usually through decades of chronic inflammation and tissue damage — much closer to Yamagiwa's irritation model than to the idea that the parasite is the tumour. The mechanisms are described on How Parasites Can Drive Cancer.

The worm today

The worm itself is still common. A 2018 genetic study examined Gongylonema neoplasticum from 114 rats of seven species in Thailand, Cambodia, Laos and the Philippines (out of archived material from almost 4,500 rodents) and describes it as a cosmopolitan parasite of rats, spread around the world with its hosts (Setsuda and colleagues, 2018). It is studied today as a rodent stomach worm, not as a cause of cancer.

How the Claim Compares With the Research

Because the presentation did not mention Fibiger, this comparison sets the speaker's general idea — that tumours are parasites, or that there is no tumour without parasites — beside what the Fibiger episode showed.

Supported

The idea that parasites can cause cancer is old and was once taken very seriously by leading scientists. It earned a Nobel Prize (Nobel Prize 1926 summary page; Tsoucalas and colleagues, 2014).
A small number of specific parasites are recognised causes of specific human cancers; Fibiger himself listed several of them in 1927.

Supported only in part

Fibiger, like the video the speaker described, reported finding parasites inside tumours. In his case the worms were real, but later research found that the growths were mostly not true cancers and that vitamin A deficiency was the main driver (Nobel Prize facts page on Fibiger; Lichtman, 2022).

Contradicted by this history

The idea that parasites are the cause of cancer in general. Fibiger, the one scientist rewarded for a parasite-cancer discovery, stated in his Nobel lecture that worms deserve "only a modest place" among the causes of human cancer and that there was "no convincing evidence" for a microbial origin of human cancer.
The idea that a tumour is a parasite. In Fibiger's own experiments the metastases "never contain either Spiroptera or eggs"; the tumour, where real, was made of the animal's own cells (Fibiger Nobel lecture, 1927).

No research found

No published study matching the presentation's description of German oncologists showing that every tumour contains parasites was identified while preparing this page.
No historical source was found linking that video to Fibiger's work. Fibiger was Danish; the German connection in his story is only that he trained under German bacteriologists and was nominated by a German pathologist.

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

What Is Not Known

Exactly what Fibiger's slides showed. Later judgements rest on repeat experiments and on re-readings of the record. Whether a few of his lesions were true invasive cancers, perhaps arising in tissue damaged by both the worm and vitamin A deficiency, is still discussed; the Nobel Prize website allows that the worm larvae caused tissue damage "where the cancer could begin developing".
How much the worm contributed at all. No modern experiment has repeated Fibiger's design with today's pathology methods and properly matched diets, so the size of any effect of Gongylonema alone is unknown.
The full committee reasoning. The nomination archive gives names and short motivations; the detailed evaluations have been studied by historians (Stolt and colleagues, 2004), but the abstracts of that work were not available for this page, so its specific conclusions are not reported here.
The source of the video in the presentation. Without the authors' names, it cannot be said whether the images shown came from a published study, and if so what that study actually concluded.

Connections

Key Research Papers

Stolt CM, Klein G, Jansson AT. An analysis of a wrong Nobel Prize-Johannes Fibiger, 1926: a study in the Nobel archives. Advances in cancer research. 2004;92:1-12. PubMed PMID: 15530554
Hitchcock CR, Bell ET. Studies on the nematode parasite, Gongylonema neoplasticum (spiroptera neoplasticum), and avitaminosis A in the forestomach of rats: comparison with Fibiger's results. Journal of the National Cancer Institute. 1952;12(6):1345-87. PubMed PMID: 14939031
Stolley PD, Lasky T. Johannes Fibiger and his Nobel Prize for the hypothesis that a worm causes stomach cancer. Annals of internal medicine. 1992;116(9):765-9. PubMed PMID: 1558350
Petithory JC, Théodoridès J, Brumpt L. [A challenged Nobel Prize: Johannes Fibiger, 1926]. Histoire des sciences medicales. 1997;31(1):87-95. PubMed PMID: 11625107
Clemmesen J. Johannes Fibiger. Gongylonema and vitamin A in carcinogenesis. Acta pathologica et microbiologica Scandinavica. Supplement. 1978;(270):1-13. PubMed PMID: 362817
Lichtman MA. Controversies in Selecting Nobel Laureates: An Historical Commentary. Rambam Maimonides medical journal. 2022;13(3):e0022. PubMed PMID: 35921488
Fujiki H. Gist of Dr. Katsusaburo Yamagiwa's papers entitled "Experimental study on the pathogenesis of epithelial tumors" (I to VI reports). Cancer science. 2014;105(2):143-9. PubMed PMID: 24313817
Tsoucalas G, Laios K, Karamanou M, Gennimata V, Androutsos G. The fascinating germ theories on cancer pathogenesis. Journal of B.U.ON. 2014;19(1):319-23. PubMed PMID: 24659685
Paliga RE. Theory on the infectious and parasitic origin of malignant diseases with reference to the polish scholars' papers - historical essay. Przeglad epidemiologiczny. 2022;76(3):421-433. PubMed PMID: 36524903
Modlin IM, Kidd M, Hinoue T. Of Fibiger and fables: a cautionary tale of cockroaches and Helicobacter pylori. Journal of clinical gastroenterology. 2001;33(3):177-9. PubMed PMID: 11500602
Weisse AB. Barry Marshall and the resurrection of Johannes Fibiger. Hospital practice (1995). 1996;31(9):105-12, 116. PubMed PMID: 8814125
Hróbjartsson A, Gøtzsche PC, Gluud C. The controlled clinical trial turns 100 years: Fibiger's trial of serum treatment of diphtheria. BMJ (Clinical research ed.). 1998;317(7167):1243-5. PubMed PMID: 9794873
Setsuda A, Ribas A, Chaisiri K, et al. Molecular genetic diversity of Gongylonema neoplasticum (Fibiger & Ditlevsen, 1914) (Spirurida: Gongylonematidae) from rodents in Southeast Asia. Systematic parasitology. 2018;95(2-3):235-247. PubMed PMID: 29446034