When microbiologist Barry Marshall wanted to prove stomach ulcers were primarily caused by the bacterium Helicobacter pylori, he did something dramatic—he drank a beaker full of the bugs. In just a few days, he started showing the first symptoms of peptic ulcer disease. Further research showed the bacteria were capable of causing stomach cancer as well, and Marshall, along with his colleague Robin Warren, earned a Nobel Prize for their efforts.
Of course, bacteria aren’t the only infectious agents capable of causing cancer, the genetic underpinnings of cancer research began with a chicken virus over a century ago. Since then, we’ve discovered several more cancers whose origins lie in infections, and even parasites that have the power to propel our cells into uncontrollable growth mode. Now, a new study published in the Lancet Oncology is offering a glimpse into what the rates of cancers caused by these pathogens looks like worldwide.
Read more: “The Trouble with Cancer Screening”
Written by an international team of cancer researchers, the report includes data from the World Health Organization’s Global Cancer Observatory for the year 2024. In that year, 12 percent of cancer cases worldwide were attributed to infectious agents, including 2.3 million new ones. H. pylori was responsible for the largest number of new cases, followed by human papilloma virus (which causes cervical cancer, among others), hepatitis B (liver cancer), and Epstein-Barr virus (which can cause several lymphomas).
It’s not the first time researchers have investigated these cancers. Five previous studies on the worldwide rates of cancer caused by pathogens have been conducted since 1990, the latest in 2018. While it appears that rates have dropped somewhat since an 18 percent high in 2002, the researchers caution that their data sources have changed over that period.
How do these infectious agents cause cancer anyway?
There are a variety of mechanisms, some of which involve our own immune systems. For example, H. pylori promotes inflammation, recruiting immune cells to the stomach lining where they can damage tissue in their attempts to kill the invader. The bacteria can also inject virulence elements into the stomach cells that disrupt adhesion and migration. As the stomach cells proliferate to heal the damage, they can turn cancerous.
Viruses, on the other hand, are a little more straightforward (after all, uncontrolled growth is their raison d’etre). They can insert themselves into our DNA, damaging tumor suppressor sequences that prevent cells from becoming cancerous, or turn them off entirely. They can also contain oncogenes (like the infamous chicken virus) that cause uncontrolled division.
The good news is that we’ve gotten pretty good at preventing infectious diseases. Between 2008 and 2022, the HPV vaccine (which celebrated its 20th birthday last June) cut the incidence of cervical precancer by a whopping 80 percent among 20- to 24-year-olds and reduced cervical cancer deaths by 62 percent, according to CDC data. And the mRNA vaccine technology created in response to the COVID-19 pandemic has accelerated vaccine development.
The threat of infections will always be with us, but with continued research, constant vigilance, and a robust public health administration, we don’t have to suffer from the cancers they cause. 
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