These 5 Infections Linked to Cancer Cause 2.3M Cases a Year

When you think about what causes cancer, smoking, alcohol, and junk food probably come to mind first. But according to a major new study from the World Health Organization's International Agency for Research on Cancer (IARC), published in The Lancet Oncology, about 1 in 8 cancers worldwide — roughly 12% of all cases diagnosed in 2024, or 2.3 million new cancer cases — were linked to an infection.

And here's the kicker: nearly all of those cases trace back to just five pathogens. Understanding infections linked to cancer is one of the biggest untapped opportunities in cancer prevention, because unlike genetics or aging, many of these infections can be prevented, treated, or vaccinated against.

The five infections behind 2.3 million cancers

1. Helicobacter pylori (H. pylori) — the stomach bacterium. This was the single biggest culprit, linked to about 4% of all cancers diagnosed in 2024, or roughly 760,000 cases. Around half the world's population carries this bacterium, usually with no symptoms at all. It spreads through contaminated food and water or close contact, and certain strains can cause chronic stomach inflammation, ulcers, and ultimately stomach cancer. Researchers say broader detection and antibiotic treatment could make a meaningful dent in global cancer rates.

2. Human papillomavirus (HPV) — the sexually transmitted virus. HPV was also linked to about 4% of all 2024 cancers — roughly 750,000 cases. High-risk HPV types cause nearly all cervical cancers, plus cancers of the throat, anus, penis, and vulva. The HPV vaccine is one of the great public-health wins of our time: Australia's vaccination program cut HPV infections by 90% in eligible groups since 2007, and the country is on track to become the first to effectively eliminate cervical cancer by 2035.

3. Hepatitis B virus (HBV). Linked to about 2% of new cancer cases, chronic hepatitis B is a leading cause of liver cancer, one of the deadliest cancers worldwide. There is a safe and effective vaccine — it is part of childhood immunization schedules in most countries — but hundreds of millions of people still live with chronic infection, many undiagnosed.

4. Epstein-Barr virus (EBV). Linked to about 1% of cancers, EBV is the newcomer on this list — the evidence that it causes cancer has grown substantially in recent years. Best known as the virus behind glandular fever (mono), which infects most people at some point, EBV is tied to certain blood cancers and nasopharyngeal cancers. No vaccine exists yet, making it a major research priority.

5. Hepatitis C virus (HCV). Linked to less than 1% of cancers, hepatitis C also drives liver cancer risk. The difference here is treatment: modern antiviral drugs can now cure hepatitis C in most people in just weeks, making liver-cancer prevention through cure a realistic goal.

How can an infection cause cancer, anyway?

It sounds strange — how does a bug in your stomach or a virus you caught as a teen lead to cancer years later? Extensive research has dissected the molecular mechanisms, and there are several routes. Some viruses insert their own DNA into the host cell's genome or produce oncogenes that hijack the cell's growth controls, effectively turning off the brakes. Others, like H. pylori, cause decades of chronic inflammation, and the constant cycle of cell damage and repair creates opportunities for dangerous mutations. Some infections also weaken the immune system's ability to spot and destroy abnormal cells.

The crucial point is timescale: these cancers typically develop years or decades after infection, which is exactly why prevention matters so much. You cannot feel cancer developing, so the window to act is while the infection is still preventable or curable.

Why Gen Z should actually care

Here's the thing: a lot of the protection against infections linked to cancer is built during young adulthood. If you are in your teens or twenties and have not gotten the HPV vaccine, it is worth asking your doctor about it — vaccination rates have actually slipped in some countries, dropping from 85.7% to 79.5% among eligible teens in Australia between 2020 and 2024, according to health officials. Catch-up vaccination in your twenties is still effective.

Beyond vaccines, the basics still apply: practice safe sex, know your hepatitis B and C status if you are at higher risk, drink safe water when traveling, and seek treatment for persistent stomach symptoms instead of ignoring them. The study also highlights an equity issue worth knowing about: around 75% of infection-related cancers occur in low- and middle-income countries, where vaccines and treatments are harder to access — a reminder that prevention tools we take for granted can be life-saving elsewhere.

The big picture is optimistic. Cancer can feel like a genetic lottery, but this research shows a huge chunk of it is actually about infections we can fight. That is a rare piece of good news in oncology: every vaccinated teenager and every cured hepatitis patient is potentially one fewer cancer case. As the study authors stress, these are population-level shares, not individual risk estimates — but they point prevention efforts exactly where they can do the most good. For more science news, see today's verified tech & science roundup at GTWebs.