WHO: Infections Cause Cancer in 1 of Every 8 Cases Worldwide

About one in eight cancer cases worldwide in 2024 — roughly 2.3 million of them — were likely caused by an infection, according to a major new study from the World Health Organization's International Agency for Research on Cancer, published September 28 in The Lancet Oncology. It is the latest large-scale evidence that infections cause cancer on a massive global scale.

The analysis found that just five pathogens account for most of these cases: Helicobacter pylori bacteria (4 percent of all cancers, about 760,000 cases), human papillomavirus or HPV (4 percent, about 750,000 cases), hepatitis B virus (2 percent, 360,000 cases), Epstein-Barr virus (1 percent, 260,000 cases), and hepatitis C virus (under 1 percent, 160,000 cases). Together, these infections are linked to more than 20 different types of cancer, including stomach, liver, blood, and cervical cancers.

The study's authors have been researching this field for around 30 years, publishing earlier estimates for 1990, 2002, 2008, 2012, and 2018. They caution that the new figures cannot be confidently compared with older ones because data sources and quality have changed over time. But the core message is clear and, in a dark way, hopeful: a huge share of the global cancer burden has identifiable, often preventable causes.

Dr. Patrick Moore, chair in Innovative Cancer Research at the University of Pittsburgh, told Newsweek the estimates may actually be "very conservative," meaning the true fraction of cancers caused by infection is likely larger. "This fraction, one in eight cancer patients, is the same fraction as lung cancer in all cancers in the U.S. It is not a rare or insignificant problem," he said.

How do infections cause cancer?

The link can sound strange — most people think of infections as short, unpleasant episodes, not as the start of a tumor. But extensive research has dissected the molecular mechanisms, and scientists now understand several ways it happens. A virus can directly or indirectly change the genes of the cell it infects, making that cell more likely to grow out of control and eventually become cancerous.

Some infections work through chronic inflammation: H. pylori, for example, is extremely common — the NHS estimates 40 percent of people in the UK are exposed — and usually causes no symptoms at all, feeling at worst like mild food poisoning. But in some people it sets off long-term inflammation in the stomach lining that can eventually turn malignant. Hepatitis B and C work similarly in the liver, where years of low-grade liver damage can end in liver cancer.

Other pathogens are more direct. HPV inserts parts of its own genetic material into host cells in a way that can switch off the cell's built-in brakes on uncontrolled growth — which is why it causes cervical cancer and a growing share of throat cancers, and another way infections cause cancer. Epstein-Barr virus, which causes glandular fever (mono), is linked to some lymphomas and nasal pharyngeal cancers, and the new study also tied it to some stomach cancers and rarer lymphomas, according to senior author Gary Clifford.

The good news: many of these cancers are preventable

Here is the part that matters most about how infections cause cancer. "These are very actionable risk factors. We can test for them. We can vaccinate against them," said Gary Clifford, the study's senior author and an epidemiologist at the International Agency for Research on Cancer. That sentence is doing real work: unlike most cancer risk factors, these are things medicine already knows how to fight.

The HPV vaccine is the headline example. Where vaccination coverage is high, cervical cancer rates are already falling. Hepatitis B vaccination of newborns is expanding across Asia, and Moore told Newsweek that expansion "may dramatically reduce liver cancers caused by this virus." H. pylori can be detected with a simple test and treated with antibiotics. Hepatitis C is now curable with antiviral drugs.

The researchers writing in The Lancet Oncology said the findings highlight "the importance of infection control" in reducing cancer rates — more evidence that infections cause cancer in millions of cases each year. For Gen Z specifically, the takeaway is practical: if you haven't been vaccinated against HPV or hepatitis B, that is a conversation worth having with a doctor. These shots are not just about avoiding an infection — they are cancer prevention with a track record.

There is also a warning tucked into the study about global health funding. Moore noted that cutbacks in HIV treatment following the disbanding of USAID "will lead to increased infectious cancers in sub-Saharan Africa" — a reminder that infections cause cancer most where prevention is scarcest, and that the places with the least prevention infrastructure carry the heaviest burden.

Cancer can feel like a random lightning strike, but infections cause cancer in millions of people every year — and testing, treatment, and vaccination are the umbrella. Read more about the research at Medical Xpress.

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