Newly Discovered Virus Linked to Colorectal Cancer

Colorectal cancer is a prevalent malignancy in the Western world and a leading cause of cancer-related deaths. Whereas age, diet, and lifestyle are established risk factors, the precise triggers for the disease often remain unclear. Recent research is increasingly focused on the gut microbiome—the complex community of bacteria, viruses, and other microorganisms residing in the digestive tract—to understand these triggers.

A Familiar Bacterium, an Unsolved Puzzle

For years, Bacteroides fragilis has been associated with colorectal cancer. However, this bacterium is also commonly found in the gut of healthy individuals, creating a paradox. Researchers at the University of Southern Denmark and Odense University Hospital investigated whether variations within the bacterium itself could explain this discrepancy.

A Virus Hidden Inside the Bacterium

The study, published in Communications Medicine, revealed that patients who later developed colorectal cancer were more likely to harbor a specific, previously undescribed virus within Bacteroides fragilis. This virus is a 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, medical doctor and Ph.D. At the Department of Clinical Microbiology at Odense University Hospital and the University of Southern Denmark.

Discovery Rooted in Danish Data

The research originated from a large Danish population study involving approximately two million citizens. Researchers identified patients with serious bloodstream infections caused by Bacteroides fragilis who were subsequently diagnosed with colorectal cancer within a few weeks. Analyzing the bacterium’s genetic material in patients with and without cancer revealed a distinct pattern: bacteria from cancer patients more frequently carried specific viruses.

Confirmed in International Datasets

To validate the initial findings, the researchers analyzed stool samples from 877 individuals with and without colorectal cancer from Europe, the United States, and Asia. They found that patients with colorectal cancer were approximately twice as likely to have traces of these viruses in their gut. “It was crucial for us to examine whether the association could be reproduced in completely independent data. And it could,” says Damgaard.

Implications for Understanding Colorectal Cancer

Up to 80% of the risk of developing colorectal cancer is linked to environmental factors, with the gut microbiome believed to be a significant contributor. The vast complexity of the gut microbiome—containing thousands of bacterial species and numerous genetic variations—has made it challenging to pinpoint specific differences between healthy individuals and those who develop the disease. “The number and diversity of bacteria in the gut is enormous. Previously, it has been like looking for a needle in a haystack. Instead, we have investigated whether something inside the bacteria—namely viruses—might help explain the difference,” explains Damgaard.

Future Directions and Potential Screening Tools

Researchers are now investigating how the virus might alter the properties of the bacterium and potentially influence the gut environment. The study suggests the possibility of developing new stool tests to identify individuals at increased risk of colorectal cancer. Preliminary analyses indicate that selected viral sequences could identify around 40% of cancer cases, while most healthy individuals did not carry them. However, the researchers emphasize that these findings are preliminary and require further investigation before they can be applied in clinical practice.

Source: Damgaard, F., et al. (2026). Distinct prophage infections in colorectal cancer-associated Bacteroides fragilis. Communications Medicine. https://doi.org/10.1038/s43856-026-01403-1