Colorectal cancer is one of the most common cancers in Western countries and a major cause of cancer-related deaths.
Doctors know that age, diet, and lifestyle all play important roles in risk. However, scientists still do not fully understand why some people develop the disease while others do not.
In recent years, researchers have focused on the gut microbiome — the huge community of bacteria, viruses, and other tiny organisms living in our intestines — as a possible clue.
A new study from researchers at the University of Southern Denmark and Odense University Hospital has uncovered a surprising discovery.
They found a previously unknown virus living inside a common gut bacterium that appears more often in people with colorectal cancer. The findings suggest that cancer risk may depend not only on which bacteria live in the gut, but also on the viruses that infect those bacteria.
The bacterium involved, called Bacteroides fragilis, has puzzled scientists for years.
It has frequently been linked to colorectal cancer, yet it is also found in many healthy people.
This contradiction led researchers to suspect that not all strains of the bacterium behave the same way. They wondered whether something inside the bacteria could explain the difference.
Their investigation revealed that in patients who later developed colorectal cancer, the bacteria were much more likely to carry a specific virus known as a bacteriophage — a virus that infects bacteria. This newly identified virus had not been described before. Scientists believe the interaction between the bacterium and the virus may influence the gut environment in ways that could be related to cancer.
The research began with data from a large Danish population study involving about two million people. Scientists looked at patients who had bloodstream infections caused by Bacteroides fragilis. Some of these patients were diagnosed with colorectal cancer shortly afterward. When researchers analyzed the bacteria from these patients, they noticed a pattern: cancer patients’ bacteria more often contained the virus.
To confirm the finding, the team examined stool samples from 877 people in Europe, the United States, and Asia. The results showed that people with colorectal cancer were about twice as likely to carry traces of the virus compared with healthy individuals. While this does not prove the virus causes cancer, the consistent pattern across countries suggests the link is real.
Researchers believe the discovery could open a new direction in understanding how colorectal cancer develops. Up to 80 percent of risk is thought to be influenced by environmental factors, including the gut microbiome. If viruses inside bacteria change how those bacteria behave, they could potentially affect inflammation, toxin production, or other processes linked to cancer.
In the future, testing for these viral markers in stool samples might help identify people at higher risk. Early results suggest the virus could help detect a portion of cancer cases that current screening methods might miss. However, scientists emphasize that the research is still in its early stages and more studies are needed before it can be used in medical practice.
This discovery highlights how complex the gut ecosystem truly is and suggests that understanding cancer may require looking not only at bacteria themselves, but also at the hidden viruses living inside
