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  5. What Parasites Are Classified as Cancer Causing by the World Health Organisation
Parasites and Disease

What Parasites Are Classified as Cancer Causing by the World Health Organisation

Lee Health Researcher
March 24, 2026 Updated: March 24, 2026 13 min read 0 comments
Medical Disclaimer This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of your doctor or other qualified health provider with any questions you may have regarding a medical condition.

Table of Contents

The World Health Organisation classifies several parasites as Group 1 carcinogens, meaning they are definitively proven to cause cancer in humans. These include Opisthorchis viverrini and Clonorchis sinensis, the liver flukes that cause bile duct cancer; Schistosoma haematobium, the blood fluke that causes bladder cancer; and Helicobacter pylori, a bacterium that causes stomach cancer. These parasites have been placed in the same category as cigarettes, asbestos, and radiation. They are not just associated with cancer. They cause cancer.

If you have ever eaten raw or undercooked freshwater fish, traveled to areas where these parasites are common, or have unexplained digestive symptoms, you could be carrying one of these cancer-causing parasites right now without knowing it. The World Health Organisation has known about these links for decades. Yet most doctors never test for these parasites. They never warn their patients about the risks. They treat the cancers that develop without ever asking what caused them.

The truth about what causes cancer has been hidden for too long. The book Cancer Is a Parasite, Not a Disease reveals how these parasites cause cancer and why the medical industry ignores this evidence.


What Are Group 1 Carcinogens

The International Agency for Research on Cancer, which is part of the World Health Organisation, classifies agents that cause cancer into several groups.

Group 1 means the agent is carcinogenic to humans. There is sufficient evidence that it causes cancer. This is the highest classification. It is reserved for agents that have been proven beyond any reasonable doubt to cause cancer in humans.

Other groups include:

  • Group 2A: Probably carcinogenic to humans
  • Group 2B: Possibly carcinogenic to humans
  • Group 3: Not classifiable
  • Group 4: Probably not carcinogenic

When an agent is placed in Group 1, it means the evidence is conclusive. The World Health Organisation does not make this designation lightly. It requires multiple studies, consistent findings, and a clear biological mechanism.

The parasites and bacteria in Group 1 are in the same category as:

  • Tobacco smoke
  • Asbestos
  • Ultraviolet radiation
  • Hepatitis B and C viruses
  • Human papillomavirus
  • Alcohol

You might also be asking why these parasites are classified as carcinogens while others are not. The designation requires strong epidemiological evidence linking the infection to cancer, as well as a clear understanding of the biological mechanism. For liver flukes, Schistosoma, and H. pylori, both conditions are met.


Liver Flukes: Opisthorchis viverrini and Clonorchis sinensis

Liver flukes are parasitic flatworms that infect the liver and bile ducts. Two species are classified as Group 1 carcinogens by the World Health Organisation.

Opisthorchis viverrini

Opisthorchis viverrini, the Southeast Asian liver fluke, is found in Thailand, Laos, Cambodia, and Vietnam. It infects an estimated 10 million people in the Mekong region alone. The infection is acquired by eating raw or undercooked freshwater fish containing the parasite larvae.

This parasite causes cholangiocarcinoma, cancer of the bile ducts. In regions where Opisthorchis viverrini is common, bile duct cancer rates are among the highest in the world. Khon Kaen Province in northeast Thailand has the highest incidence of this cancer globally. The link is so strong that it cannot be ignored.

The parasite causes cancer through chronic inflammation, mechanical damage to the bile ducts, and toxic secretions. The worms attach to the bile duct walls, feed on tissue, and release waste products. Over years and decades, this constant damage leads to cellular mutations and eventually cancer.

Clonorchis sinensis

Clonorchis sinensis, the Chinese liver fluke, is found in China, Korea, Vietnam, Taiwan, and parts of Russia. It infects an estimated 15 to 20 million people worldwide. Like Opisthorchis viverrini, it is acquired by eating raw or undercooked freshwater fish.

Clonorchis sinensis also causes cholangiocarcinoma. It was classified as a Group 1 carcinogen by the World Health Organisation in 2009. The mechanism is the same as for Opisthorchis viverrini. The worms live in the bile ducts for decades, causing chronic inflammation, scarring, and cellular changes that lead to cancer.

How Liver Flukes Cause Cancer

The process takes years or decades. The worms attach to the bile duct walls using suckers. They feed on tissue and blood. Their eggs become trapped in the bile ducts, causing inflammation. Your immune system forms granulomas around the eggs. The constant cycle of damage and repair causes cells to divide repeatedly. Each division carries a risk of mutation. After many years, some cells become cancerous.

The book Cancer Is a Parasite, Not a Disease explains in detail how liver flukes cause cancer and why doctors rarely test for them.


Schistosoma haematobium

Schistosoma haematobium is a parasitic flatworm that causes urogenital schistosomiasis. It is classified as a Group 1 carcinogen by the World Health Organisation.

Geographic Distribution

Schistosoma haematobium is found in sub-Saharan Africa, the Middle East, and parts of South America. An estimated 200 million people are infected with schistosomiasis globally, with Schistosoma haematobium being one of the most common species.

The infection is acquired by contact with freshwater contaminated with the parasite larvae. When you swim, wade, or bathe in contaminated water, the larvae burrow through your skin and travel to the blood vessels around your bladder.

Cancer Caused

Schistosoma haematobium causes squamous cell carcinoma of the bladder. In regions where this parasite is common, bladder cancer rates are dramatically higher. In Egypt, before schistosomiasis control programs, bladder cancer was the most common cancer in men and the second most common in women. The vast majority of these cases were linked to Schistosoma infection.

How Schistosoma Causes Bladder Cancer

The female worms lay eggs that become trapped in the bladder wall. Your immune system forms granulomas around each egg. The eggs secrete toxic substances that damage cells. The constant inflammation causes the bladder lining to change from transitional cells to squamous cells, a process called squamous metaplasia. This change is protective in the short term but increases cancer risk over time.

After years of chronic infection, some cells undergo mutations and become cancerous. The cancer is typically squamous cell carcinoma, which is different from the transitional cell carcinoma more common in Western countries.

Decline in Cancer Rates with Control

When schistosomiasis control programs were implemented in Egypt, the rate of Schistosoma infection declined. As infection rates fell, bladder cancer rates also fell. This provides powerful evidence that the parasite causes cancer. Eliminate the parasite, and you eliminate the cancer.


Helicobacter pylori

Helicobacter pylori is a bacterium, not a parasite, but it is included in the World Health Organisation’s list of Group 1 carcinogens alongside parasites. It is often discussed in the same context because it shares the same mechanisms of chronic infection and inflammation leading to cancer.

Geographic Distribution

H. pylori infects about half the world’s population. In many countries, more than 50 percent of people carry this bacterium. It is most commonly acquired in childhood and can persist for a lifetime if not treated.

Cancer Caused

H. pylori causes gastric adenocarcinoma, cancer of the stomach, and gastric MALT lymphoma, a cancer of lymphoid tissue in the stomach. It is responsible for the majority of stomach cancers worldwide.

How H. pylori Causes Cancer

H. pylori infects the stomach lining and causes chronic gastritis. The bacterium produces an enzyme called urease that neutralizes stomach acid, allowing it to survive. Certain strains carry a gene called CagA that produces a toxin that damages stomach cells.

The chronic inflammation causes changes in the stomach lining called atrophic gastritis and intestinal metaplasia. These are precancerous conditions. Over decades, some cells become cancerous.

The link between H. pylori and stomach cancer is so well established that the World Health Organisation classified it as a Group 1 carcinogen in 1994. Treatment of H. pylori infection reduces stomach cancer risk.

The book Cancer Is a Parasite, Not a Disease explains the connection between H. pylori and stomach cancer in detail.


Other Parasites and Infectious Agents Linked to Cancer

While only certain parasites are classified as Group 1 carcinogens, other infectious agents are also linked to cancer.

Human Papillomavirus (HPV)

HPV is a virus classified as a Group 1 carcinogen. It causes cervical cancer, anal cancer, and other cancers. Vaccines are available to prevent HPV infection.

Hepatitis B and C Viruses

Hepatitis B and C are viruses classified as Group 1 carcinogens. They cause liver cancer. Vaccines are available for hepatitis B.

Epstein-Barr Virus

Epstein-Barr virus is a Group 1 carcinogen. It causes Burkitt lymphoma, nasopharyngeal carcinoma, and other cancers.

Human T-cell Lymphotropic Virus Type 1

HTLV-1 is a Group 1 carcinogen. It causes adult T-cell leukemia and lymphoma.

Kaposi Sarcoma Herpesvirus

KSHV is a Group 1 carcinogen. It causes Kaposi sarcoma.

Schistosoma mansoni and S. japonicum

While S. haematobium is classified as a Group 1 carcinogen for bladder cancer, the intestinal species S. mansoni and S. japonicum have been linked to colorectal cancer. The evidence is not as strong, so they are not classified as Group 1.

Strongyloides stercoralis

Strongyloides has been linked to colorectal cancer in some studies, but the evidence is not sufficient for Group 1 classification.

Cryptosporidium

Cryptosporidium has been linked to gastrointestinal cancers in immunocompromised individuals, but the evidence is limited.


Why Doctors Don’t Test for These Parasites

Despite the clear evidence that these parasites cause cancer, most doctors never test for them. Here is why.

Lack of Awareness

Many doctors are not aware that parasites can cause cancer. They were not taught this in medical school. They assume cancer is caused by genetics, lifestyle factors, or bad luck.

Geographic Bias

Doctors in Western countries assume these parasites are only a problem in developing countries. This is not true. Travelers can bring these infections home. Immigrants can carry them for decades. People can be infected without ever leaving their home country.

Inadequate Testing

The standard tests for these parasites are often inaccurate. Stool tests can miss liver fluke eggs. Urine tests can miss Schistosoma eggs. Blood tests may not distinguish between past and current infection.

Lack of Clinical Suspicion

Doctors do not think to test for parasites unless a patient has traveled to an endemic area and has specific symptoms. Even then, many doctors are not familiar with the testing options.

Profit Motives

There is no profit in testing for and treating parasites. The drugs are cheap. The tests are not profitable. The cancer industry makes far more money treating the cancers that develop years later.


Symptoms of Parasitic Infections That Cause Cancer

The symptoms of these infections are often vague and easily dismissed. Many people have no symptoms at all for years or decades.

Liver Fluke Symptoms

  • Abdominal pain, especially in the upper right side
  • Bloating and indigestion
  • Nausea
  • Diarrhea or constipation
  • Fatigue
  • Jaundice in advanced cases
  • Unexplained weight loss

Schistosoma haematobium Symptoms

  • Blood in the urine
  • Frequent urination
  • Painful urination
  • Pelvic pain
  • Infertility in advanced cases

H. pylori Symptoms

  • Burning stomach pain
  • Bloating
  • Nausea
  • Loss of appetite
  • Dark or black stools from bleeding ulcers

If you have any of these symptoms, especially if you have traveled to endemic areas, you need to consider the possibility of parasitic infection.

The book Cancer Is a Parasite, Not a Disease provides detailed information on how to identify these infections and what to do about them.


How These Infections Are Diagnosed and Treated

If you suspect you have one of these infections, you need to be proactive. Doctors will not always test for them unless you ask.

Liver Fluke Diagnosis and Treatment

Diagnosis is made by finding eggs in stool, blood tests for antibodies, or imaging showing bile duct changes. Treatment is with praziquantel, a single dose or two-day course. The cure rate is excellent.

Schistosoma Diagnosis and Treatment

Diagnosis is made by finding eggs in urine, blood tests for antibodies, or imaging showing bladder changes. Treatment is with praziquantel, a single dose. The cure rate is excellent.

H. pylori Diagnosis and Treatment

Diagnosis is made by urea breath test, stool antigen test, or endoscopy with biopsy. Treatment is with a combination of antibiotics and acid-suppressing medications for 14 days.


What You Can Do Right Now

If you have ever traveled to areas where these parasites are common, eaten raw or undercooked freshwater fish, or have unexplained digestive or bladder symptoms, you need to take action.

Step 1: Read the Book

The first step is to educate yourself. Cancer Is a Parasite, Not a Disease contains the information you need to understand which parasites cause cancer and how to protect yourself. This book reveals the truth that the medical industry does not want you to know.

Step 2: Get Tested

Ask your doctor for parasite tests. Be specific. Tell them about your travel history and your symptoms. If your doctor refuses, find a tropical medicine specialist or a functional medicine practitioner who understands parasitic infections.

Step 3: Get Treated

If you test positive, treatment is simple and effective. Antiparasitic drugs like praziquantel can eliminate these infections and reduce your cancer risk.

Step 4: Support Your Body

After treatment, your body needs to heal. Support your liver, bladder, or stomach with proper nutrition, clean water, and gentle detox methods.

Step 5: Stay Informed

The information in this article is just the beginning. Cancer Is a Parasite, Not a Disease goes much deeper. It reveals the full story of how parasites cause cancer and how you can protect yourself and your loved ones.


FAQ

What parasites are classified as cancer causing by the World Health Organisation?

The World Health Organisation classifies Opisthorchis viverrini, Clonorchis sinensis, and Schistosoma haematobium as Group 1 carcinogens. Helicobacter pylori, a bacterium, is also classified as Group 1.

What type of cancer do liver flukes cause?

Liver flukes cause cholangiocarcinoma, cancer of the bile ducts. Opisthorchis viverrini and Clonorchis sinensis are both proven to cause this cancer.

What type of cancer does Schistosoma haematobium cause?

Schistosoma haematobium causes squamous cell carcinoma of the bladder.

What type of cancer does H. pylori cause?

H. pylori causes gastric adenocarcinoma and gastric MALT lymphoma.

How do these parasites cause cancer?

These parasites cause cancer through chronic inflammation, physical damage to tissues, toxic secretions, and immune suppression over many years or decades.

Where are these parasites found?

Liver flukes are found in Asia. Schistosoma haematobium is found in Africa, the Middle East, and South America. H. pylori is found worldwide.

How do you get these infections?

Liver flukes are acquired by eating raw or undercooked freshwater fish. Schistosoma is acquired by contact with contaminated freshwater. H. pylori is acquired through oral-oral or fecal-oral transmission.

Can these infections be treated?

Yes, all of these infections can be treated. Liver flukes and Schistosoma are treated with praziquantel. H. pylori is treated with antibiotics.

Why don’t doctors test for these parasites?

Doctors are not trained to look for parasites, assume they are only a problem in developing countries, and rely on inaccurate tests. There is also no profit in testing and treating parasites.

What are the symptoms of liver fluke infection?

Symptoms include abdominal pain, bloating, nausea, fatigue, and jaundice in advanced cases. Many people have no symptoms for years.

What are the symptoms of Schistosoma infection?

Symptoms include blood in the urine, frequent urination, painful urination, and pelvic pain. Many people have no symptoms for years.

What are the symptoms of H. pylori infection?

Symptoms include burning stomach pain, bloating, nausea, and loss of appetite. Many people have no symptoms.

Can these infections be prevented?

Yes. Do not eat raw or undercooked freshwater fish. Avoid contact with freshwater in areas where Schistosoma is common. Practice good hygiene to prevent H. pylori.

Where can I learn more about parasites that cause cancer?

Read Cancer Is a Parasite, Not a Disease for comprehensive information on these parasites and how to protect yourself.


The World Health Organisation has spoken. Liver flukes cause bile duct cancer. Schistosoma haematobium causes bladder cancer. H. pylori causes stomach cancer. These are not theories. They are established facts backed by decades of research. These parasites are classified as Group 1 carcinogens, in the same category as cigarettes and asbestos.

Yet the medical industry continues to ignore this evidence. Doctors do not test for these parasites. They do not warn patients about the risks. They treat the cancers that develop without ever asking what caused them.

Why? Because there is no profit in preventing cancer. There is no profit in testing for and treating cheap parasitic infections. The money is in treating cancer with expensive drugs, radiation, and surgery.

You have a choice. You can wait for cancer to develop and trust a system that profits from treating it. Or you can educate yourself, get tested, and take control of your health.

The information you need is available in Cancer Is a Parasite, Not a Disease. This book reveals the truth about the parasites that cause cancer. It explains how to get tested. It provides the protocols for treatment. And it shows you how to protect yourself and your loved ones.

Do not wait until it is too late. These parasites can live inside you for decades, silently causing damage that leads to cancer. Take action today. Get tested. Get treated. And read the book that could save your life.

Tags: Group 1 carcinogens H. pylori infectious causes of cancer liver flukes parasite cancer link Schistosoma WHO carcinogens
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