Second, a virus can insert its own DNA into the host cell's DNA. This insertion can accidentally disrupt genes that control cell death or cell division.

Third, some viruses carry a cancer-causing gene — an oncogene — often picked up by accident from another organism.

This line of research has been foundational to modern cancer science. Studying tumor viruses helped scientists establish the direct link between genetics and cancer that we now take for granted.

Bacteria and parasites play a role too

Viruses are not the only culprits. Chronic infections with bacteria or parasites — such as H. pylori or Opisthorchis viverrini, a liver fluke (a type of parasitic worm) — can also lead to cancer, though more indirectly.

Long-term infection places significant stress on tissues. Inflammation, normally a helpful part of the immune response, can become overactive and damage tissue further. This stress can cause cancer-causing DNA damage in cells, or produce mistakes during cell division as the body rapidly makes new cells to repair the damage. Either way, cells acquire genetic mutations that push them toward becoming cancerous.

There may still be unknown mechanisms connecting infections and cancer. But the connection itself, the authors stress, is indisputable.

A preventable slice of the burden

The practical message of the study is that many of these cancers are preventable.

Australia offers one of the clearest examples. Since the national HPV vaccination program began in 2007, HPV infections have dropped by 90% among people eligible for the vaccine. The vaccine protects against strains of the virus strongly associated with cervical cancer and is offered to adolescents. Thanks to this success, Australia could be on track to eliminate cervical cancer by 2035.

The trend since then is less encouraging. Vaccination rates among 15-year-olds fell from 85.7% in 2020 to 79.5% in 2024. That matters because, as with all vaccines, a high proportion of the population must be immunized to protect people who cannot be vaccinated for health reasons. The WHO and Australian target is to vaccinate 90% of 15-year-old girls by 2030.

The study also reveals a stark global divide: around 75% of infection-caused cancers occurred in low- and middle-income countries.

Treating infections that can lead to cancer — such as HIV, H. pylori, and hepatitis B and C — is one way to cut cancer rates. Preventive measures, including vaccination, condoms and disease screening, play a major role as well. But the availability of these programs in poorer countries is often limited, making access a central equity issue in the fight against cancer.

The findings come with caveats. The estimates describe likely attribution rather than proven causation in individual patients, and changing data quality limits comparisons with earlier studies. Even so, the overall picture is clear: a meaningful share of the world's cancer cases trace back to infections that medicine already knows how to prevent, treat or screen for.