Colorectal cancer: a hidden intestinal virus may double the risk

By Elodie Vaz | Published on March 6, 2026 | 3 min read


Colorectal cancer is among the most common cancers in Western countries and represents a major cause of cancer-related mortality. While age, diet, and lifestyle are well-established risk factors, the biological mechanisms that trigger the disease remain insufficiently understood in most cases. Up to 80% of the risk is thought to be linked to environmental factors, among which the intestinal microbiome—comprising bacteria, viruses, and other microorganisms—is increasingly considered a key element in etiological hypotheses.  

A very common… but suspicious bacterium  

Among the bacteria frequently associated with the disease, Bacteroides fragilis holds a particular position. It is often detected in patients with colorectal cancer but is also present in the vast majority of healthy individuals.  

“It is paradoxical to observe that the same bacterium is consistently associated with colorectal cancer while also being an integral part of the intestinal microbiota in healthy people,” explained Flemming Damgaard, MD, PhD in clinical microbiology at Odense University Hospital and the University of Southern Denmark, and the study’s lead author, in a press release.  

What If the real culprit were a hidden virus?  

Researchers from the University of Southern Denmark and Odense University Hospital hypothesized that intra-species differences could explain this association. Their objective was to determine whether a specific genetic element—particularly viral—present within Bacteroides fragilis might be correlated with colorectal cancer. The results were published in Nature Communications Medicine on February 7.  

The research began in Denmark using a large population-based study including approximately two million citizens. Researchers identified patients who had experienced severe bloodstream infections caused by Bacteroides fragilis. Among them, a subgroup was diagnosed with colorectal cancer several weeks later.  

When viruses and bacteria work together  

“We discovered a virus that had not been described before and that appears to be closely linked to the bacteria found in patients with colorectal cancer,” explained Flemming Damgaard.  

Initial observations, based on a limited number of Danish samples, allowed the researchers to formulate a hypothesis that was subsequently tested using independent international datasets.  

External validation included 877 individuals with or without colorectal cancer from Europe, the United States, and Asia, using fecal samples.  

A future tool to identify at-risk individuals?  

Analyses showed that patients with colorectal cancer were about twice as likely to carry traces of this bacteriophage in their intestinal microbiota. “It was important for us to examine whether this association could be reproduced using completely independent data—and it was,” said Flemming Damgaard.  

This represents a robust statistical association observed across several countries, although it does not establish causality. “We still do not know whether the virus contributes to the cause of the disease or whether it simply indicates that something else has changed in the intestine,” he noted.  

Researchers emphasize that “it is not only the bacterium itself that appears to be important. It is the bacterium interacting with the virus it carries.” The identified bacteriophage may represent a previously undescribed viral type.

Preliminary analyses suggest that a set of viral sequences could identify approximately 40% of colorectal cancer cases, while most healthy individuals do not carry them.  

This study proposes a new interpretation of the link between the microbiome and colorectal cancer. Beyond the mere presence of bacteria, the interaction between bacteria and viruses could represent a key determinant. The complexity and diversity of the microbiome make the identification of discriminating factors particularly challenging.  

“The number and diversity of intestinal bacteria are enormous. Previously, identifying them was like searching for a needle in a haystack. We therefore investigated whether something inside the bacteria—the viruses—could explain the difference,” explained Flemming Damgaard.  

At this stage, the clinical implications remain exploratory. “In the short term, we can investigate whether this virus could be used to identify individuals at increased risk,” he said. In the longer term, detecting specific bacteriophages in stool samples could complement current screening strategies.

Ongoing research will aim to determine whether this virus actively contributes to carcinogenesis or simply represents a biomarker of an altered intestinal ecosystem. The challenge is substantial: transforming a microbiological correlation into a mechanism-based understanding—or even into a predictive tool.

Read next: Colorectal cancer: early screening changes the game and enables earlier action

About the Author – Elodie Vaz
Health journalist, CFPJ graduate (2023).
Élodie explores the marks diseases leave on bodies and, more broadly, on human life. A registered nurse since 2010, she spent twelve years at patients’ bedsides before exchanging her stethoscope for a notebook. She now investigates the links between environment and health, convinced that the vitality of life cannot be reduced to that of humans alone.