Ask what causes cancer and the answers come back like an old catechism: cigarettes, sunlight, bad luck in the bloodline. Almost nobody names the germ. Yet a team at the International Agency for Research on Cancer, the World Health Organization’s cancer arm in Lyon, has now put a figure to the overlooked enemy. Writing in The Lancet Oncology on Monday, they estimate that 2.3 million of the new cancer cases recorded worldwide in 2024 — 12 per cent, one in every eight — were caused by viruses, bacteria and other infectious agents, according to reporting by CBC News and Scientific American.
The ledger of microbes
At the top of the ledger sits not a virus but a bacterium: Helicobacter pylori, discovered only in the early 1980s, carried often since childhood, blamed for some 760,000 cases, predominantly stomach cancers in eastern Asia. Close behind comes human papillomavirus, the cause of 100 per cent of cervical cancers and of anal, vulvar, vaginal, penile and some head and neck cancers — nearly 750,000 cases, with high rates in sub-Saharan Africa. Together those two accounted for more than 1.5 million cases, Scientific American notes. Hepatitis B was linked to an estimated 360,000 cases and, with hepatitis C, to a significant share of liver cancers.
The study updates earlier work from 2018, and the science has moved. Researchers added 16 cancers to the list of those tied to infection. “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,” Gary Clifford, a cancer epidemiologist and study co-author based in Lyon, told CBC. Among the additions: HIV’s link to cervical cancer, and a rare virus, Merkel cell polyomavirus, behind an aggressive skin cancer.
The largest change in arithmetic belongs to Epstein-Barr virus, the near-universal passenger most people carry without knowing it. It was linked to about 260,000 cases in 2024 — 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. The virus is also a key risk factor for multiple sclerosis, and unlike HPV or H. pylori it has no vaccine and no drug. “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.”
A map drawn by money
The report’s geography reads like an audit of history. Where states invested decades ago in screening and vaccination, the infectious cancers receded; where they did not, they remain. High-income countries fare far better than lower- and middle-income ones, a gap the researchers attribute to historical disparities — or, as Clifford put it to Scientific American, to “disparities in investment,” such as cervical screening programs that exist in the United States but not across much of sub-Saharan Africa.
The bitter irony is that these are among the most preventable of all cancers. “These are very actionable risk factors. We can test for them. We can vaccinate against them,” Clifford said. Hepatitis C can be treated, hepatitis B vaccinated against, H. pylori eradicated with two weeks of antibiotics. The HPV vaccine, introduced in the US in 2006, prevents six cancers; in some American states, cervical cancer rates among women aged 20 to 31 — the first cohort offered the shot — have fallen by more than 50 per cent. “There is no question that this vaccine does what it was intended to do,” said Anna Giuliano of the Moffitt Cancer Center. Clifford’s summary has the ring of an epitaph for excuse-making: “serious mortal diseases can be prevented with a simple population-based equality-driven jab in the arm.” Yet he concedes: “We really still have a lot of work to do implementing things that we know work.”
The Canadian exception, and its cracks
Canada sits on the fortunate side of the ledger. A 2019 study led by Karena Volesky, then at McGill University, put infections’ share of Canadian cancers at about 4 per cent — still more than 7,000 diagnoses a year. Even here the machinery is stalling. Cervical cancer rates, long declining under childhood vaccination, 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,” said Elizabeth Holmes of the Canadian Cancer Society. The national plan demands 90 per cent vaccination and 90 per cent screening; only about five provinces exceed 80 per cent coverage, Holmes said, and at least one sits as low as 66 per cent.
And H. pylori crosses borders without a passport. About 2,000 Canadians die of stomach cancer each year; Dr. Paul Moayyedi, a gastroenterologist at McMaster University, calls it “a horrible disease to die from” and wants universal screening — a blood or breath test, stool sample or endoscopy, cheap against a two-week course of antibiotics. The bacterium follows crowded housing and poor childhood sanitation, and Moayyedi argues high-risk groups, including First Nations communities, should at least be offered testing. “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.