Is Cancer a Parasite?

Cancer is not a parasite. And despite the claims you may have seen online and in social media, there’s no evidence that breast cancer is caused by a parasite. 

The myth dates all the way back to the late 1800s, when scientists — buoyed by the discovery of the bugs behind many illnesses, like tuberculosis — thought cancer, too, could be caused by an organism that came from outside the body. The theory fell through when researchers weren’t able to “infect” anyone with cancer. The myth has resurfaced over the years, even so. 

Unlike parasites, cancer starts inside the body, with cells that grow and divide abnormally. But both parasites and cancer can cause damage to your body and make you very sick, so they may seem similar to many people.

“When people use the word parasite in everyday conversation, they mean something that grows in you or on you, and takes energy from you,” says Daniel O’Neil, MD, MPH. O’Neil is an associate professor of medicine at Yale School of Medicine and breast medical oncologist at Yale Cancer Center and Smilow Cancer Hospital in Connecticut. “If you use that broad definition, it's easy to understand how people could think about cancer and breast cancer as being similar to a parasite,” even if it’s not, he says.

A parasite is an organism that lives on or in another organism, called a host. Parasites depend on the host for food and other resources. Parasites can harm your body by stealing its nutrients and by causing illness or infection in the process. 

There are three main types of parasites that cause disease in people: protozoa, helminths, and ectoparasites. 

Protozoa are too small to be seen without a microscope. Some protozoa live and multiply in the intestines. They’re spread when food or water is contaminated with infected feces (poop). For example, last summer’s cyclosporiasis outbreak linked to lettuce, was caused by the parasite Cyclospora. The parasite can infect the lining of the intestines and lead to diarrhea, cramps, vomiting, and fatigue. 

Helminths are a type of parasitic worm, including roundworm and tapeworm. Depending on the type of worm, they can live in the gastrointestinal tract, blood, or in other tissues. People can become infected by eating or drinking contaminated food or water or through contact with contaminated soil. 

Ectoparasites, including ticks, fleas, lice, and mites, attach to the outside of the body. Some, like mosquitoes, can spread many different types of diseases through their bites. Some of those diseases can be deadly. 

Cancer cells are not a separate organism, says O’Neil. “They ultimately originate from our own body,” he says.

Cancer starts when cells in the body begin to grow and divide out of control.

Normally, cells grow and divide when the body needs new ones. When cells become damaged or are no longer needed, they get a signal to die. This helps keep the body healthy.

Cancer cells don’t follow these rules. Changes in their genes can cause them to keep growing and dividing, even when the body doesn’t need them. Over time, these extra cells can build up and form a tumor. 

There are a few isolated studies that have looked at a possible link between a parasite called Toxoplasma gondii (T. gondii) and breast cancer, says O’Neil.

T. gondii causes toxoplasmosis, one of the most common parasitic infections in the world. Nearly one-third of people have been exposed to this parasite, usually from eating undercooked meat. For most healthy adults, toxoplasmosis doesn’t cause major symptoms. But the infection can be serious for people with weakened immune systems, like people with AIDS. Toxoplasmosis also can cause severe complications in pregnancy, raising the risk of miscarriage and stillbirth. It’s the reason why pregnant women are told to avoid eating undercooked meat and eggs, and exposure to cat litter. 

One 2025 study found a high rate of antibodies against T. gondii (which indicate an earlier infection with the parasite) in samples from people with breast cancer. But researchers did not show that T. gondii played a role in developing breast cancer. Another laboratory study looked into whether T. gondii may help the immune system recognize cancer and attack it. These early findings didn’t suggest that parasitic infections could treat cancer, but researchers are hopeful that studying the links between parasites and cancer could lead to the development of new cancer treatments. 

There’s not enough research to say that T. gondii or any parasite causes cancer or treats it, says O’Neil.

One study published in the International Journal of Cancer found a possible link between T. gondii and the risk of a rare type of brain cancer. Researchers noted that people with higher exposure to T. gondii were more likely to develop glioma, a highly fatal type of cancer.  

Additionally, the International Agency for Research on Cancer (IARC), a World Health Organization agency, has identified three parasitic worms that are known causes of cancer — but not breast cancer — in humans. Schistosoma haematobium, a freshwater parasite found in some parts of Asia, Africa, and the Middle East, is linked to a higher risk of bladder cancer. Opisthorchis viverrini and Clonorchis sinensis, which are also freshwater parasites, can cause infections that increase the risk of bile duct cancer. 

Claims about cancer being a parasite may have led to the idea that anti-parasitic drugs, like ivermectin and fenbendazole, could be cancer treatments. 

A handful of studies on ivermectin, which is FDA approved to treat certain parasitic infections in people, suggest that it may disrupt cancer cell growth in certain ways, says O’Neil. However, most of those findings come from studies of cell lines and mice, he adds. “That's very different from showing that the drug can treat cancer in people.” And often, the dose of ivermectin used in studies is much higher than the doses that are safely given to people, he adds.

There is a clinical trial currently underway in nine people with metastatic triple-negative breast cancer to evaluate the effectiveness of taking ivermectin along with the anti-cancer drug balstilimab. It’s a phase I/II trial, which means the focus is on understanding safety, dosage, effectiveness, and side effects of these medicines in a small group of people before moving on to later phases that compare the drug combo to other available treatments and track the long-term effects. “And in that setting, ivermectin was being given with another active anti-cancer agent, so it wasn't being taken by itself,” O’Neil emphasizes. The hope was that ivermectin would increase sensitivity to this immunotherapy drug, he says. “It’s difficult to say anything about how well the ivermectin worked in this one small trial,” he adds. 

The American Society of Clinical Oncology emphasizes that there is “no robust, peer-reviewed clinical evidence demonstrating that either ivermectin or fenbendazole is safe or effective for treating any human malignancy.”