Hidden virus in gut bacteria linked to doubled colorectal cancer risk

Researchers in Denmark have identified a previously unknown virus living inside a common gut bacterium, and the finding could reshape how doctors screen for and prevent colorectal cancer. The virus, a bacteriophage that infects bacteria rather than human cells, was discovered hiding within Bacteroides fragilis—a microorganism that normally inhabits the human digestive tract. What makes the discovery significant is this: colorectal cancer patients were roughly twice as likely to carry traces of this virus compared to people without cancer, according to a study published in Communications Medicine.

The research team, led by Dr. Flemming Damgaard of Odense University Hospital and the University of Southern Denmark, analyzed stool samples from 877 individuals across Europe, the United States, and Asia. The virus type they identified had never been documented before. For years, doctors have known that Bacteroides fragilis appears more frequently in people with colorectal cancer, but since most healthy people also carry the bacteria, the reason it seemed harmful in some cases and benign in others remained a puzzle. By examining the genetic material within the bacteria itself, the researchers uncovered something unexpected: a complete viral genome nested inside.

Damgaard described the surprise of the finding plainly. The team had not anticipated discovering a virus within the bacteria when they began their work. The bacteriophage—a virus that targets bacteria, not human cells—represents a new piece of the colorectal cancer puzzle, though researchers are careful to note what they do not yet know. The study establishes a strong statistical association between the virus and cancer, but it does not prove the virus causes colorectal cancer. Co-author Ulrik Stenz Justesen acknowledged the major limitation: scientists still cannot explain why the virus is linked to the disease. The team is now conducting laboratory experiments and animal studies to determine whether the virus changes how the bacterium behaves in ways that might influence cancer development.

Colorectal cancer ranks among the leading causes of cancer-related deaths worldwide, according to the World Health Organization. Recent high-profile deaths—including those of actor James Van Der Beek, Catherine O'Hara, Kirstie Alley, Pelé, and Chadwick Boseman—have underscored the disease's reach across age groups. While diet and lifestyle account for much of the risk, researchers increasingly believe the bacteria living in the gut play an important role in cancer development. The human body contains roughly as many microbial cells as human cells, a fact that underscores how deeply microbes are woven into human health.

The potential practical applications are what make this discovery compelling. Current colorectal cancer screening relies on stool tests that detect invisible blood and on colonoscopies. In the future, doctors might be able to test stool samples for the viral markers identified in this study, offering a new screening tool. Researchers pointed to cervical cancer as a model for how identifying a viral cause can accelerate prevention. After HPV was linked to cervical cancer, vaccines helped drive down new cases significantly. While it is too early to draw that parallel for colorectal cancer, the pathway is clear: if a definitive viral role is confirmed, prevention strategies could follow. Damgaard emphasized the early stage of the work, saying it is too early to act on these findings. What researchers want people to understand now is the hope the discovery represents—and the many health-related discoveries still waiting to be made within the human microbiota.