The first section of this book discusses Clostridium difficile Infections (CDI). There is no consensus on the definition of severe CDI, since it varies depending on the authors or clinical practice guidelines consulted. Severe CDI can be defined as an episode of CDI with one or more specific signs and symptoms of severe colitis or a complicated clinical course, with significant systemic toxicity and shock, requiring ICU admission or a colectomy, or resulting in death. The chapters in this section examine the risk factors for the development of severe CDI; study the advances in fecal microbiota transplantation; and provide management strategies of CDI in intensive care units. Section Two of this book provides research on Klebsiella pneumoniae infections. Klebsiella pneumoniae producing KPC-type carbapenemase (KPC-Kp) is an emerging problem epidemiologically, diagnostically, and therapeutically. Carbapenem-resistant Klebsiella pneumoniae have emerged as a class of pathogens that pose a significant threat to patients admitted to healthcare facilities. The first chapter of this section reviews the molecular epidemiology of Klebsiella pneumoniae carbapenemase. The following chapters discuss KPC-Kp; Klebsiella pneumoniae brain abscess in adults; and the causes of Ankylosing spondylitis and Crohn's disease.