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Exposure to a common stomach bug, which more than half the global population will have at some point in their lives, has been linked with colorectal cancer in a new study.
Researchers at the University of California San Diego and the University of Hong Kong found that exposure to the bacteria Helicobacter pylori (H. pylori) was associated with a higher risk of colorectal cancer. The study was published in the journal eGastroenterology on September 8.
The bacteria has been associated with other cancers and diseases in the past, including stomach cancer, and in 1994, it was classified as a human carcinogen by the World Health Organization’s International Agency for Research on Cancer (IARC).
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"Colorectal cancer is a multifactorial disease influenced by age, genetics, lifestyle, diet, and many other factors," Dr. Shailja Shah, one of the study leads and professor of medicine at the University of California, San Diego, told Newsweek.
"Our study cannot establish causation—that is, we cannot say for certain that H. pylori directly causes colorectal cancer, though we know this is true for stomach cancer. Instead, our study shows an epidemiological association."
Dr. Wael El-Rifai, associate director of basic science with Sylvester Comprehensive Cancer Center, who was not involved in the study, told Newsweek the findings represent "a significant step forward, given the global burden of both H. pylori infection and colorectal cancer."
"While earlier studies offered limited preliminary insights, this is arguably the most comprehensive and compelling evidence to date," he said.
Excluding skin cancers, colorectal cancer is the third most common cancer diagnosed in both men and women in America, according to the American Cancer Society. The cancer is expected to cause over 55,000 deaths in the U.S. during 2026.
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What Is H. Pylori?
H. pylori is a bacteria that causes a stomach infection, and it is believed more than 50 percent of the global population will be exposed to it at some point in their lives.
These infections often happen during childhood, and tend to spread through contact with an infected person's bodily fluids, including vomit, stool or saliva, or through contaminated food and water.
The infections can cause stomach pain, bloating, gas and other symptoms, but most people who have the infection don't have symptoms. In some cases, the infection can result in sores called peptic ulcers in the lining of the stomach or small intestine.
The bacteria is becoming less common across the globe, Dr. Martin Blaser, a professor of medicine and pathology and laboratory medicine at Rutgers University, who was not involved in the study, told Newsweek.
As it declines, he added, "so is stomach cancer," which he said was "consistent with [H. pylori's] known major role in stomach cancer."
How Can H. Pylori Cause Cancer?
The researchers found that H. pylori exposure was associated with a 1.59-fold increase in the risk of colorectal cancer and they estimated that it could be related to around 22 percent of global cases of the cancer. Although, this does not mean that for 22 percent of cases the bacteria caused the cancer, Shah said.
While more needs to be done to determine whether there is a causal link between the bacteria and colorectal cancer, El-Rifai said that such a link has been "firmly established" for stomach cancer.
The connection is believed to be because H. pylori "primarily colonize the stomach, causing localized, chronic inflammation in the gastric lining," he said, adding that "its downstream effects extend far beyond."
He said it can "trigger a strong chronic inflammatory response in the gastric mucosa, with systemic immune response, flooding the bloodstream with inflammatory signaling molecules (cytokines and chemokines)."
"When these inflammatory chemokines and cytokines reach the colon, they can disrupt the normal balance of colonic tissue, promoting rapid cell growth and activating pro-survival pathways that could allow abnormal cells to thrive," he said.
He added that an H. pylori infection can "alter the gut microbiome, creating a microenvironment that may favor tumor development in the lower gastrointestinal tract."
What The Findings Could Mean
Shah said: "I would be cautious about suggesting any immediate change to colorectal cancer screening practice based on our findings alone," adding that she thought "our study raises an important research question: if the association is eventually shown to be causal, and if H. pylori eradication is shown to reduce colorectal cancer risk, then infection could potentially represent an additional modifiable factor in colorectal cancer prevention."
She said that this could be "particularly relevant in populations where H. pylori remains very common."
"These findings—specifically, that it is estimated that one-fifth of all colorectal cancer cases are due to exposure to the common bacteria H. pylori—have the potential to be incredibly impactful, as H. pylori can be successfully eradicated with a 2-week antibiotic treatment, suggesting a feasible and cost-effective colorectal cancer interception strategy," Meira Epplein, a professor of population health sciences and medicine at Duke University, who was not involved in the study, told Newsweek.
"The next step towards implementation would be to clearly define high-risk populations to apply this strategy to, to avoid mass screening, with its potential for increasing antibiotic resistance," she said.
Reference
Li Y, Cui C, Sun D, Ho K, Qian C, Guo Y, et al. (2026) Global and regional burden of colorectal cancer potentially related to Helicobacter pylori exposure: a risk attribution modelling study. eGastroenterology. https://doi.org/10.1136/egastro-2026-100436
Contact Newsweek editors on this story: Marc Vargas and Emma Lee-Sang
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