Cancer can seem random. It’s often the result of many factors, including genetic mutations that build up over a lifetime of exposure to sun, pollution, and other sources of DNA damage, combined with a person’s genetic predisposition. DNA damage accumulated over time can weaken the body’s ability to suppress out-of-control cell growth, which is how tumors form.

But infections are another significant cause of cancer. Uncovering the links between certain infections and cancers raises the hope that by preventing infection, one could prevent certain malignancies. A recent report from the World Health Organization’s International Agency for Research on Cancer (IARC) published in the Lancet Oncology found that 12% of new cancer cases in 2024—the most recent year for which data were available—were caused by 12 infections. Which infections are associated with cancer?

Certain human papillomaviruses, or HPV, are behind the majority of cervical cancers and a number of head and neck cancers as well, says Harriet Rumgay, an epidemiologist at IARC who is an author of the new report. About 4% of new cancer diagnoses in 2024 could be attributed to HPV infection. Doctors and researchers have known for decades that hepatitis B and hepatitis C infections also drive many cases of liver cancer, linked to 2% and less than 1% 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 gastric cancers. This infection was calculated to be behind about 4% of new diagnoses. And Epstein-Barr Virus (EBV), which causes mononucleosis and affects more than 90% of the world’s population, was involved in around 1% of new diagnoses, including lymphomas, nasopharyngeal cancers, and gastric cancers. Seven other infections, including HIV, were also flagged as carcinogens in the study.

Most people who have these infections will not develop cancer. It’s not also immediate—there may be a years-long delay. And infection is often only one of a number of factors contributing to a given cancer. For instance, when it comes to the relatively high rates of EBV-driven nasopharyngeal cancers in China, researchers believe that some genetic predisposition and environmental factors are also involved. Why do some infections cause cancer?

It depends on the infection. In the case of HIV, the virus suppresses the immune system, making it easier for other carcinogenic viruses to replicate. H. pylori causes tissue damage and produces toxins that destabilize the body’s natural control of cellular growth and division, which can lead to cancer.

With EBV, says Claire Shannon-Lowe, a virologist at the University of Birmingham in the U.K. who studies EBV and cancer but was not involved in the research, it has to do with how the virus hides within immune cells called B cells. “It's a whole host of things that the virus itself is doing to the B cell to make it continually grow,” she says. That includes blocking normal cellular housecleaning procedures that would both root the virus out and arrest cancer development. How can these cancers be prevented?

Some of these infections have treatments. H. pylori can be cleared with antibiotics, for example, reducing the risk of gastric cancer, and treating HIV reduces cancer risk from the infection as well. Some other infections on the list have vaccines.

In fact, for some of these vaccines, cancer prevention was a major goal driving their development. “The hepatitis B virus vaccine was the first vaccine that was developed against cancer,” says Rumgay. Likewise, the HPV vaccine, which was approved 20 years ago, is intended to prevent cervical cancer. Cancer epidemiologists may start seeing the effects of widespread HPV vaccination in future analyses, Rumgay says. “More and more people who have been vaccinated are starting to reach the age where we would potentially see the impact on cervical cancer rates or other cancer rates,” she says.

However, these vaccines aren’t making it to everyone who might benefit from them. In the U.S., hepatitis B vaccination rates in newborns have declined steeply over the last two years.

For other infections, no vaccines are available. For EBV, a nearly ubiquitous virus with a long, clearly documented history of contributing to cancer, “we don't have an effective vaccine yet, or an effective way of preventing it,” says Rumgay. “This is something that we want to highlight with our study.”Historically, it was difficult to interest pharmaceutical companies in vaccines for viruses like EBV that are common and nonlethal, says Lawrence Young, a virologist and emeritus professor of molecular oncology at 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 in part to research suggesting EBV might be involved in multiple sclerosis, an autoimmune disorder, “they've all become very interested,” he says. A number of vaccine candidates are now being studied, with some in clinical trials.

For some of the infections implicated by the latest study—the Kaposi-sarcoma associated herpes virus, in particular—there’s need for much more research into prevention, says Rumgay. The hope is that “these new findings might stimulate more development in that area,” she says.