Newly discovered virus linked to colorectal cancer

Scientists have found that a newly discovered virus that infects a particular bacterium in the gut may be linked to colorectal cancer. – Image generated by Wix AI

CANCER DIGEST – May 24, 2026 – Scientists  have discovered a new and previously undescribed virus

that infects gut bacteria in patients with colorectal cancer. 

Led by Flemming Damgaard, PhD, and a molecular biologist at the University of Southern Denmark, the research team analyzed data from a large Danish population study involving around two million citizens. Their study appeared in the journal Communications Medicine last Feb. 7, 2026.

The investigators were looking for a particular bacterium that has been the center of a paradox concerning colorectal cancer. The bacterium called bacteroides fragilis is often found in patients diagnosed with colorectal cancer. The paradox is that many healthy people without cancer also have the bacterium in their guts. Consequently, the researchers decided to see if there were any differences in the bacteroides fragilis found in colorectal cancer patients and the bacteroides fragilis in non-cancer patients.

What they found was that in patients with the bacterium who later developed cancer, the bacterium itself had been infected with a virus that had not previously been described. Such viruses, called bacteriophages, can alter the bacterium’s behavior. Following further analysis, the researchers found that bacteria from cancer patients were more often infected with specific viruses. While this did not prove a link, it provided a statistical signal indicating there could be a link.

That finding was based on a relatively small sample drawn from exclusively Danish patients. To see if this potential link might be present in a broader population, they analyzed stool samples of 877 people from Europe, the US and Asia with and without colorectal cancer. What they found was that colorectal cancer patients were twice as likely to have traces of these viruses in their gut.

When they did genetic analyses of several of the viruses they were able to identify the colorectal cancer patients 40 percent of the time. This suggests that further study will be needed to develop a more precise marker of cancer for clinical use. 

To do that the researchers have grown bacteroides fragilis that carry the virus in the lab in order to study how the bacterium is altered by the virus and how that changes the interaction with gut tissue. If the virus alters the properties of the bacterium, it could potentially change the gut environment. 

The long-term goal is to be able to test for these viruses in stool samples to assess the risk of colorectal cancer and potentially improve prevention and treatment.

Sources: University of Southern Denmark press release