Abstract. Clostridium difficile is a spore-forming, anaerobic, Gram-positive bacillus responsible for 15% to 25% of cases of post-antibiotic diarrhea and more than 95% of cases of pseudomembranous colitis. It is the leading cause of nosocomial infectious diarrhea in adults. Since 2003, the incidence and proportion of severe forms of C. difficile infections have increased significantly in Canada and the United States. This trend is believed to be linked to the emergence and spread of a specific C. difficile clone known as 027, based on its PCR-ribotyping profile, or "NAP1," based on its pulsed-field gel electrophoresis profile. The increased virulence of this clone is thought to be due to the overproduction of toxins A and B and/or the production of a third toxin called the binary toxin. This clone is also characterized by its resistance to erythromycin and the newer fluoroquinolones (moxifloxacin, levofloxacin). First detected in North America, it has spread to Great Britain, Belgium, the Netherlands, and more recently to France, where it has been responsible for several outbreaks in northern France. This concerning trend requires increased vigilance in healthcare facilities to control its spread early on. This vigilance relies on the systematic screening for C. difficile in all patients presenting with nosocomial diarrhea, on reporting clusters of cases and severe forms of infection to the DDASS and the CCLIN, and on the characterization of strains by expert laboratories organized around the National Reference Center for Anaerobes.