For many people, sun exposure, smoking and family history are the first thoughts that come to mind regarding cancer risk factors. While infections are rarely high on the list, new research shows one in eight cancers worldwide are actually caused by them.
These viruses, bacteria and other microbes are the focus of a new paper published Monday in The Lancet Oncology that linked an estimated 2.3 million new cancer cases in 2024 — 12 per cent of all new cases — to these infectious agents.
A team of researchers at The International Agency for Research on Cancer (IARC), an arm of the World Health Organization, conducted the study. They found the largest number of cancer cases attributed to infections globally were caused by the bacterium Helicobacter pylori, which can increase the risk of stomach cancer and accounted for 760,000 cases, predominantly in eastern Asia.
Human papillomavirus (HPV) followed close behind. HPV, which causes 100 per cent of cervical cancer cases and also anal, vulvar, vaginal, penile and some head and neck cancers, accounted for nearly 750,000 new cases globally, with high rates in sub-Saharan Africa. Hepatitis B virus (HBV) and hepatitis C virus (HCV) were also linked to a significant number of liver cancer cases.
For the first time, researchers linked even more cancers to infections, expanding their list to include 16 additional ones.
Gary Clifford, a cancer epidemiologist and study co-author based in Lyon, France, said the science has advanced significantly.
“There’s been new infectious agents identified in this time as being cancer-causing, and even the infectious agents that we knew about in the past, the evidence has increased about the number of different cancers that they can cause,” he said.
Some of those new infectious agents included HIV and its link to cervical cancer, and a very rare virus called Merkel cell polyomavirus that can lead to an aggressive form of skin cancer.
To come up with numbers, researchers first examined which infections cause cancer, then calculated what fraction of these cancers can be linked to specific infections and applied those calculations to global databases of new cancer cases.
Another significant shift is the recognition of the role of Epstein-Barr virus (EBV). EBV was linked to about 260,000 cases globally in 2024, and can lead to nasopharyngeal cancer, some gastric cancers and Hodgkin’s lymphoma.
“That’s quite a big burden of cancer caused by Epstein-Barr virus that was not counted in the past, but is now recognized,” Clifford said.
Epstein-Barr virus is also a key risk factor in developing multiple sclerosis, and unlike some of the other infections that have vaccines or antibiotic treatments, there are no available tools to fight against it.
“The fact that we’re seeing Epstein-Barr has caused all these chronic diseases — it’s increasingly becoming a target that people need to focus on for prevention,” he said.
The global report highlights vast regional differences, mostly driven by historical disparities, researchers said. High-income countries that established cancer screening and vaccination programs over the last few decades are doing much better compared to lower- and medium-resourced countries where access to prevention and treatment is limited.
“We really still have a lot of work to do implementing things that we know work and we’ve known for a long time but haven’t really made it into all populations in the world,” Clifford said.
In North America, the burden of cancer-causing infections is much lower, but H. pylori and HPV are the most common sources.
Infections causing cancer in Canada
In Canada, according to a 2019 study, the rate of cancers attributable to infections was estimated to be at four per cent. Karena Volesky, an epidemiologist in Montreal, led the Canadian research team while at McGill University.
“It tells us how important each of the 12 infections are,” Volesky said of the new global numbers.
While there is a lower prevalence of these forms of infections in Canada, her study estimated these infections lead to more than 7,000 cancer diagnoses each year.
Doctors say Canada is facing a silent health crisis of HPV-related cervical cancer that could be prevented if more people got vaccinated against human papillomavirus, the virus that can cause the disease.
“HPV is certainly an infection that we’re keeping an eye on in Canada,” said Elizabeth Holmes, the director of health policy at the Canadian Cancer Society.
Holmes said cervical cancer rates had been steadily declining in Canada due to robust childhood vaccination programs, but plateaued last year.
“That’s certainly a concern — that we might not meet our goal of elimination of cervical cancer by 2040 unless those rates start to decline again,” she said.
Human papillomavirus is transmitted through sexual contact, but the HPV vaccine protects against the most common forms of infection that can cause cancer.
Canada’s cervical cancer elimination plan aims to increase vaccination rates to 90 per cent, increase screening rates to 90 per cent and ensure timely followup care after screening.
“Right now, only about five provinces are above 80 per cent, and I think at least one is as low as 66 per cent,” Holmes said.
The stomach bacteria that can lead to cancer
The infection leading to the most cancer cases is Helicobacter pylori, a bacterium first discovered in the early 1980s that mainly leads to gastric cancers. While the research found the highest burden in eastern Asia in countries like Japan, South Korea and China, it’s a problem here, too.
“It’s a horrible disease to die from,” said Dr. Paul Moayyedi, a leading researcher and gastroenterologist at McMaster University. About 2,000 Canadians die every year from stomach cancer.
Moayyedi wants to see a universal screening program for the bacterium, which can be detected through a blood or breath test, stool sample or endoscopy and treated with a combination of antibiotics over a two-week period.
Helicobacter pylori is associated with crowded living conditions and poor sanitation in childhood. While most people with it can live a long life without any issues, if left untreated, it can cause ulcers and stomach cancer for some.
Moayyedi said stomach cancers may not be seen as high priority compared to other types of cancers, but believes screening programs for those at high risk, such as people with a family history of stomach cancers or First Nations communities, would save lives.
“In consultation with the elders of local communities it would be important to at least offer those groups screening if they would want it,” he said.
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