Pseudomembranous colitis is an acute inflammatory disease of the colon that in mild cases may appear as minimal inflammation or edema of the colonic mucosa. In more severe cases, the mucosa often is covered with loosely adherent nodular or diffuse exudates. These raised exudative plaques are 2-5 mm in size. Coalescence of these plaques generates an endoscopic appearance of yellowish pseudomembranes lining the colonic mucosa.
In the late 1800s, before the availability of antibiotics, Finney reported the first case of pseudomembranous colitis, calling it "diphtheritic colitis."  Hall and O'Toole first described Clostridium difficile in 1935.  C difficile was first implicated as a causative factor in pseudomembranous colitis in the 1970s. Over the past 100 years, pseudomembranous colitis has changed from a fatal disease caused by a postoperative event to, in the era of antibiotics, a commonly occurring complication of antibiotic use that may lead to serious morbidity but that usually is treated easily. 
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Pseudomembranous colitis is an inflammatory disease of the colon. In some cases (5-19%), the disease will be localized to the cecum and the proximal colon.
The antibiotic-induced change in the balance of normal gut flora allows overgrowth of C difficile.  Colitis results from the bacterial production of large amount of toxins. The most important toxins are toxin A (enterotoxin) and toxin B (cytotoxin). One theory explains the variable severity of clinical disease through differential production rates of one toxin or the other by selected bacterial isolates. Most experts, however, believe that these variations are due to host factors.
The toxins bind to the mucosa, attack the membranes and the microfilaments of the mucosal cells, and subsequently result in cytoplasmic contraction, hemorrhage, inflammation, cellular necrosis, and protein loss. They also interfere with mucosal protein synthesis, stimulate granulocyte chemotaxis, and increase capillary permeability, intestinal myoelectric responses, and peristalsis. Intestinal tissue invasion by C difficile has been reported in fatal cases of pseudomembranous colitis in pediatric patients with hematologic malignancy.
Pseudomembranous colitis usually is associated with antibiotic use, which may alter the balance of normal gut flora and allow overgrowth of certain organisms.  Clindamycin, lincomycin, ampicillin, and cephalosporin have been implicated in most of the reported cases, but any antimicrobial agent (including antifungal, antiviral, and metronidazole) could incite the disease, regardless of the amount administered or the route of administration.
C difficile, a gram-positive, spore-forming, anaerobic bacillus, is isolated in almost all of these cases. Rare cases have been related to Staphylococcus aureus, Salmonella species, Clostridium perfringens, Yersinia species, Shigella species, Campylobacter species, cytomegalovirus, Entamoeba histolytica, and Listeria species.
Conditions other than antimicrobial administration could predispose to C difficile pseudomembranous colitis. Such conditions include bowel ischemia, recent bowel surgery, uremia, dietary change, change in bowel motility, malnutrition, chemotherapy, shock, and Hirschsprung disease.
The incidence of antibiotic-associated diarrhea ranges from 5% to 39%, depending on the antibiotic type. Pseudomembranous colitis complicates 10% of cases of antibiotic-associated diarrhea. C difficile is found in the stool of 15-25% of asymptomatic, antibiotic-treated, hospitalized adults. Similar numbers were found in debilitated patients and in patients who received one dose of prophylactic antibiotics before surgical procedure.
C difficile is an unusual component of healthy bowel flora. It is found in 3-5% of healthy adults; however, as many as 50% of infants and children harbor the bacteria and its toxins. Pseudomembranous colitis is a surprisingly rare disease in infants and young children—a population recognized as frequent asymptomatic colonizers. The low incidence of colitis in the pediatric population is attributed to the strength of the immune system. Antibodies to C difficile frequently are detected in infected young patients.
About 25% of human C difficile isolates are nontoxigenic. C difficile colitis is the fourth most common nosocomial disease reported to the Centers for Disease Control and Prevention. C difficile is one of the most frequently isolated enteric pathogens, second only to Campylobacter jejuni. 
High-risk populations  include elderly people, patients in the intensive care unit (ICU), people with uremia, people with burns, people undergoing abdominal surgery, women undergoing cesarean delivery, and patients with cancer. One suggestion is that these patients do not have greater susceptibility to the disease but are at heightened risk of nosocomial infection. C difficile can be transmitted nosocomially, via the hands of personnel or by contaminated objects. It can survive in spore form for as long as 5 months on hospital floors.
The overall mortality for pseudomembranous colitis is 2%. The mortality in untreated elderly or debilitated patients is 10-20%. Even with surgical intervention, mortality and morbidity in patients with toxic megacolon remain high (35% and 66%, respectively).