Hidden virus in your gut may be linked to colon cancer
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Hidden virus in your gut may be linked to colon cancer
A hidden virus in gut bacteria may be the missing link behind colorectal cancer risk.
Date:
April 22, 2026
Source:
University of Southern Denmark Faculty of Health Sciences
Summary:
A newly discovered virus hiding inside a common gut bacterium could help explain one of medicine’s long-standing mysteries: why a microbe found in both healthy people and cancer patients is linked to colorectal cancer. The research suggests that the interaction between bacteria and the viruses they carry may be key to understanding disease risk. It may even lead to future screening tests that detect cancer risk earlier.
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FULL STORY
A newly identified virus inside gut bacteria is strongly linked to colorectal cancer, offering a surprising new clue about the disease. The discovery hints that viruses within microbes—not just the microbes themselves—could play a critical role. Credit: Shutterstock
Colorectal cancer is among the most common cancers in Western countries and remains a major cause of cancer-related deaths. While factors like age, diet, and lifestyle are known to influence risk, the exact triggers behind the disease are still not fully understood.
In recent years, scientists have increasingly focused on the gut microbiome, the vast ecosystem of bacteria, viruses, and other microorganisms living in the digestive system.
Now, researchers from the University of Southern Denmark and Odense University Hospital have identified a previously unknown virus inside a common gut bacterium. This virus appears more often in people with colorectal cancer, offering a new clue about how the disease may develop.
A Common Gut Bacterium With a Longstanding Mystery
For years, researchers have linked one specific bacterium, Bacteroides fragilis, to colorectal cancer. However, this connection has been difficult to explain because the same bacterium is also found in most healthy individuals.
"It has been a paradox that we repeatedly find the same bacterium in connection with colorectal cancer, while at the same time it is a completely normal part of the gut in healthy people," says Flemming Damgaard, medical doctor and PhD at the Department of Clinical Microbiology at Odense University Hospital and the University of Southern Denmark.
To resolve this contradiction, the team investigated whether there might be important differences within the bacterium itself.
They found that there were.
Discovery of a Virus Inside Gut Bacteria
The key difference turned out to be a virus living inside the bacterium. In patients who later developed colorectal cancer, Bacteroides fragilis was much more likely to carry a specific bacteriophage, a virus that infects bacteria.
"We have discovered a virus that has not previously been described and which appears to be closely linked to the bacteria we find in patients with colorectal cancer," says Flemming Damgaard.
The researchers believe this virus represents entirely new types that have not been identified before.
"It is not just the bacterium itself that seems interesting. It is the bacterium in interaction with the virus it carries," he explains.
Although the study shows a strong statistical link between the virus and colorectal cancer, it does not prove that the virus causes the disease.
"We do not yet know whether the virus is a contributing cause, or whether it is simply a sign that something else in the gut has changed," says Flemming Damgaard.
Large-Scale Data Reveals a Clear Pattern
The discovery began with data from a large Danish population study involving about two million people. Researchers focused on patients who had experienced serious bloodstream infections caused by Bacteroides fragilis. A small portion of these individuals were diagnosed with colorectal cancer within weeks.
By comparing bacterial samples from patients with and without cancer, the team identified a clear pattern. Bacteria from cancer patients were more likely to contain specific viruses.
The initial findings came from a relatively small group of Danish samples, but they provided a strong starting point for further investigation.
"It was in our Danish material that we first detected a signal. That gave us a concrete hypothesis, which we were then able to investigate in larger datasets," says Flemming Damgaard.
Confirmed Across Nearly 900 Patients Worldwide
To test whether the pattern held up globally, the researchers analyzed stool samples from 877 individuals across Europe, the United States, and Asia.
The results were consistent. People with colorectal cancer were about twice as likely to carry these viruses in their gut.
"It was important for us to examine whether the association could be reproduced in completely independent data. And it could," says Flemming Damgaard.
While this strengthens the link, it still does not show that the virus directly c