These infections are linked to 12% of new cancer cases

A recent report shows that pathogens play a significant role in many cancer cases.

Cancer can seem random. It is often the result of many factors, including genetic mutations that develop over a lifetime of sun exposure, pollution, and other sources of DNA damage, along with a person's genetic predisposition. Accumulated DNA damage over time can weaken the body's ability to suppress uncontrolled cell growth, which is how tumors form.

But infections are another important cause of cancer. The discovery of the connection between some infections and cancers gives hope that by preventing infections, some malignancies can be prevented. A recent report by the World Health Organization's International Agency for Research on Cancer (IARC) published in Lancet Oncology found that 12 percent of new cancer cases in 2024—the most recent year for which data are available—were caused by 12 infections.

Which infections are associated with cancer?

Some human papillomaviruses, or HPVs, are behind most cervical cancers and some head and neck cancers, says Harriet Ramgee, an epidemiologist at IARC who is an author of the new report. About 4% of new cancer diagnoses in 2024 can be attributed to HPV infection.

Doctors and researchers have known for decades that hepatitis B and hepatitis C infections also cause many cases of liver cancer, which are associated with 2 percent and less than 1 percent of new diagnoses, respectively, according to the report.

There is also strong evidence linking Helicobacter pylori, a bacterium that infects the stomach and small intestine, with stomach cancer, accounting for about 4 percent of new diagnoses. And Epstein-Barr virus (EBV), which causes mononucleosis and affects more than 90% of the world's population, has been implicated in about 1% of new diagnoses, including lymphoma, nasopharyngeal cancer, and gastric cancer. Seven other infections, including HIV, were also identified as carcinogens in this study.

Most people who have these infections do not develop cancer. It is also not immediate - there may be a delay of several years. And infection is often only one of a number of factors contributing to the development of a given cancer. For example, when it comes to the relatively high rate of nasopharyngeal cancers caused by EBV in China, researchers believe that some genetic background and environmental factors are also involved.

It depends on the infection. In the case of HIV, the virus suppresses the immune system and makes it easier for other cancer-causing viruses to multiply. Helicobacter pylori causes tissue damage and produces toxins that destabilize the body's normal control of cell growth and division, which can lead to cancer.

Claire Shannon-Lowe, a virologist at the University of Birmingham in the UK who studies EBV and cancer but was not involved in the research, says that EBV has to do with how the virus hides in immune cells called B cells. "It's a series of things that the virus itself does to the B cell to keep growing," he says. That includes blocking the cell's natural cleaning processes that both eradicate the virus and stop the cancer from growing.

Some of these infections are treatable. Helicobacter pylori can be cleared with antibiotics, for example, reducing the risk of stomach cancer, and HIV treatment also reduces the risk of cancer caused by the infection. Some of the other infections on the list have vaccines.

In fact, for some of these vaccines, cancer prevention was the primary goal of their development. "The hepatitis B virus vaccine was the first vaccine developed against cancer," says Rumgay. Likewise, the HPV vaccine, approved 20 years ago, is intended to prevent cervical cancer. Cancer epidemiologists may look at the effects of widespread HPV vaccination in future analyses, Ramgi says. "More and more people who are vaccinated are getting to an age where we will potentially see an effect on cervical cancer rates or other cancer rates," he says.

However, these vaccines are not available to everyone who might benefit from them. In the United States, hepatitis B vaccination rates among infants have fallen sharply over the past two years.

For other infections, no vaccine is available. For EBV, a nearly ubiquitous virus with a long and well-documented history of causing cancer, "we still don't have an effective vaccine or an effective way to prevent it," says Ramgi. "That's what we want to highlight with our study."

Historically, drug companies have been interested in vaccines for viruses like EBV that are common and non-lethal, says Dr. Baudelaires Young, a virologist and professor emeritus of molecular oncology at the University of Warwick in the U.K. who has spent his career studying cancer and EBV. (Young was not involved in the IARC research.)

But thanks to research showing that EBV may play a role in multiple sclerosis, an autoimmune disorder, "they're all getting very interested." A number of vaccine candidates are currently being studied, some in clinical trials.

Much more prevention research is needed for some of the infections implicated in the latest study -- particularly Kaposi-sarcoma-related herpesvirus, Rumgay says. The hope, he says, is that "these new findings may stimulate further development in that area."