Researchers from the University of Southern Denmark and Odense University Hospital have identified a previously unknown virus, a bacteriophage, residing within the common gut bacterium Bacteroides fragilis, which may be a significant contributor to colorectal cancer. This discovery, published in "Communications Medicine," emerged from analyzing data from a Danish study of two million citizens, noting a correlation between severe B. fragilis bloodstream infections and subsequent colorectal cancer diagnoses. Further genetic analysis of bacteria from cancer patients and healthy individuals revealed a higher prevalence of this novel virus in those with cancer. International data sets confirmed that individuals with colorectal cancer were twice as likely to have traces of these viruses, suggesting their potential as a biomarker for early detection and opening avenues for new treatment and prevention strategies.