Hidden virus inside gut bacteria linked to doubled colorectal cancer risk, study finds

A newly discovered virus hiding inside common gut bacteria may be linked to colorectal cancer, according to new research.

Scientists in Denmark found that colorectal cancer patients were about twice as likely to carry a previously unidentified virus within Bacteroides fragilis, a bacterium that normally lives in the human gut, according to the study published in Communications Medicine earlier this month.

Researchers say the discovery could help scientists better understand the role of the gut microbiome in cancer development.

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"These findings highlight the importance of the microorganisms in the gut and their relation to our health," Dr. Flemming Damgaard, PhD, of the Department of Clinical Microbiology at Odense University Hospital and the University of Southern Denmark, told Fox News Digital. "If we want to understand the full picture, we need to look deep into their genetic material."

Doctors have known for years that Bacteroides fragilis shows up more often in people with colorectal cancer, which includes cancers of the colon and rectum, but since most healthy people also carry the bacteria, it wasn’t clear why it would be harmful in some cases and not others.

So researchers looked at subtle genetic differences within the bacteria and uncovered something unexpected.

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"We were very surprised to find a whole virus inside the bacteria from the colorectal cancer patients," Damgaard said. "That was not something we expected when we started our study."

The virus, known as a bacteriophage, infects bacteria rather than human cells. According to the researchers, the viral type they identified had not previously been documented.

When the team analyzed stool samples from 877 individuals across Europe, the United States and Asia, they found that colorectal cancer patients were approximately twice as likely to carry traces of the virus compared to people without cancer.

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The findings show a strong statistical association, but researchers stress that the study does not prove the virus causes colorectal cancer, and it is too early to draw immediate conclusions.

"The major limitation is that we still don’t know why the virus is linked to colorectal cancer," said co-author Ulrik Stenz Justesen. "But we are already continuing our research."

The team is now conducting laboratory experiments and animal studies to determine whether the virus alters the bacterium’s behavior in a way that could influence cancer development.

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Studies have found that the human body contains roughly as many microbial cells as human cells, underscoring how deeply microbes are intertwined with human health.

"Our understanding of [microbial cells] is still in its infancy," Damgaard said. "There are many health-related discoveries to be made in the human microbiota."

Colorectal cancer is one of the leading causes of cancer-related deaths worldwide, according to the World Health Organization. Several high-profile celebrity deaths, including James Van Der Beek, Catherine O'Hara, Kirstie Alley, Pelé and Chadwick Boseman, have drawn attention to colorectal cancer’s impact across age groups in recent years.

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While much of colorectal cancer risk is tied to diet and lifestyle, researchers think the bacteria in our gut may also play an important role.

Currently, colorectal cancer screening includes stool tests that check for blood that isn’t visible to the naked eye, as well as colonoscopies. Researchers say that in the future, it may be possible to test stool samples for viral markers like those identified in the study.

Researchers pointed to cervical cancer as an example of how identifying a viral cause can help prevention efforts. After HPV was linked to cervical cancer, vaccines helped drive down new cases. 

While it's too early to make that comparison, they said colorectal cancer could one day follow a similar path if a clear viral role is confirmed.

"It is too early to act," Damgaard said. "We have a lot of hope, and that is what we want people to have at this point."

Fox News Digital's Melissa Rudy contributed reporting.