Clostridium tertium is an aerotolerant, endospore-forming bacterium in the family Clostridiaceae notable among clostridia for its ability to grow in the presence of atmospheric oxygen, despite being classified within the genus of strictly anaerobic organisms. Its relative aerotolerance distinguishes it from most genus members and contributes to its occasional isolation from clinical sources. C. tertium is considered an opportunistic pathogen, causing bacteremia primarily in immunocompromised patients including those undergoing chemotherapy, with neutropenia as a key risk factor. The bacterium has been isolated from soil, water, and the gastrointestinal tract of animals and humans. Despite causing bacteremia and occasionally more invasive infections, C. tertium does not produce recognized exotoxins comparable to the major histotoxic clostridia, and its pathogenic mechanisms involve host immune evasion rather than direct toxin-mediated tissue destruction. Clinically, C. tertium bacteremia carries significant mortality in neutropenic patients but responds to beta-lactam antibiotics unlike C. difficile or C. perfringens infections. Its natural aerotolerance makes it unusual within the Clostridium genus and raises evolutionary questions about the anaerobic ancestry of this lineage.