The Infectious Clue: Linking Bacterial Infections to Underlying Malignancies

Emerging evidence suggests some bacterial infections may be early signs of hidden cancers rather than random events. Yet this link remains under-recognized in practice, representing an often-missed diagnostic opportunity. Large registry studies show that certain infections are linked to a sharply increased short-term risk of cancer detection, with most of the excess diagnoses clustering in the first 6 months after the index episode. Key associations include the following: (i) anaerobic or gut-derived bacteremia with <i>Bacteroides</i>, <i>Clostridium</i>, <i>Fusobacterium</i>, or pks⁺<i>Escherichia coli</i> before colorectal neoplasm; (ii) <i>Streptococcus gallolyticus/bovis</i> bacteremia and colorectal neoplasm; (iii) cryptogenic <i>Klebsiella pneumoniae</i> liver abscess and pancreaticobiliary or colorectal cancer; (iv) non-resolving pneumonia and segmental collapse before lung cancer. Overall, short-term cancer detection risks range from about 3% after unselected Gram-negative bacteremia to ~8% or higher after cryptogenic liver abscess-similar to accepted thresholds that justify targeted cancer work-up. Even hidden tumors disrupt immunity, compromise barriers, and create conditions that favor microbial invasion. This review synthesizes evidence for the "sentinel infection phenotype"; outlines pathogen-specific associations, including their possible pathogenetic mechanisms; and proposes a practical diagnostic framework. Recognizing these infection signatures may enable earlier cancer detection and better outcomes.