The most common causes of diarrhoea include: 

• Bacterial Infections such as E. coli (“traveler’s diarrhoea”), Salmonella, Campylobacter jejuni (campylobacteriosis), and Yersinia enterocolitica (yersiniosis). Other infectious causes include rotavirus (rotavirus gastroenteritis), Norwalk virus (viral gastroenteritis), adenoviruses, astroviruses, cryptosporidium parvum (cryptosporidiosis), and microscopic parasites called microsporidia (microsporidiosis).
• Viral infections such as rotavirus, norovirus (causes gastroenteritis), and enteric adenoviruses.
• Food-borne infections from toxins or pathogens from foods prepared by an infected food handler. Such foods may be contaminated with staphylococcus aureus, salmonella, campylobacter jejuni, Shigella sp., Escherichia coli O157:H7 (“E. coli”), and others. Common agents of this type of diarrhoea include staphylococcal enterotoxin B and C; toxic strains of Escherichia coli that produce Shiga toxin; and Clostridium perfringens epsilon toxin.
• Food allergies or sensitivities, lactose intolerance, fructose intolerance, gluten-sensitive enteropathy.
• Irritable bowel syndrome, inflammatory bowel disease including Crohn’s disease and ulcerative colitis. Other causes of chronic diarrhoea include fecal incontinence, anorectal diseases such as proctitis cystica profunda, rectal prolapse, anal fissure, or stricture. Iatrogenic causes are drugs for treating other disorders that may cause diarrhoea as a side effect.
• Radiation enteritis—in this condition, the large intestine is irritated by radiation therapy.
• Changes in intestinal flora due to broad-spectrum antibiotics that disturb normal gut microbiota allowing opportunistic bacteria to overgrow, resulting in bacterial overgrowth syndrome.
• Inadequate absorption of carbohydrates such as lactose, fructose, or sorbitol due to a lack or absence of digestive enzymes that normally break these sugars down into absorbable monosaccharides. Such a deficiency can be genetic (congenital sucrase-isomaltase deficiency) or secondary to damage to the pancreas from autoimmune disease, infection, or metabolic disorder.
• Intestinal parasites such as giardia lamblia (“beaver fever”) and Strongyloides stercoralis commonly cause diarrhoea, as does ingesting food contaminated with ova of intestinal parasites. Chemical agents may also cause diarrhoea by damaging the lining of the intestine increasing peristalsis, or affecting the intestine’s nerve endings.
• Chronic laxative abuse can irritate or damage the bowel’s lining resulting in chronic diarrhoea.
• Anal fissure, anal fistula, rectal polyps, and proctitis are common causes of rectal-anorectal diarrhoea.
• Hirschsprung disease is a condition where there is the absence of ganglia cells in that section of the intestine, which results in poor muscle contractions or peristalsis. 
• Amyloidosis is a group of diseases resulting from deposits of an abnormal protein called amyloid in body tissues. The most common type, primary amyloidosis, results from the buildup of disease-causing proteins made by the bone marrow and/or lymph nodes. Tissues become enlarged, stiff and may cause diarrhoea.
• Chagas’ disease (American trypanosomiasis) is caused by “Trypanosoma cruzi,” an organism transmitted to humans by blood-sucking triatomine bugs found mainly in South America. This parasitic infection can affect many organs but causing chronic colitis with bloody stools; this condition responds very well to topical antiparasitic drugs like benznidazole.
• Fulminating or c difficile colitis is an acute form of toxic megacolon (massive colon enlargement). It is caused by the bacterium “Clostridium difficile,” producing toxins A and B, which can cause severe inflammation in the lining of the large intestine. C difficile colitis must be treated promptly with aggressive antibiotic therapy to avoid complications including colon perforation, massive bleeding from ulceration, sepsis, and death. 
• Crohn’s disease is a chronic inflammatory disorder that may affect any part of the gastrointestinal tract but most commonly affects the small intestine’s terminal ileum. The symptoms of Crohn’s disease may range from abdominal pain to diarrhoea, fever, and even weight loss. The pathophysiology involves a shift in the way the immune system responds to bacteria and other antigens leading to chronic inflammation.
• Ulcerative colitis is a form of inflammatory bowel disease (IBD) that specifically affects only the large intestine (colon) and frequently, but not always, occurs in association with Crohn’s disease or indeterminate colitis. Symptoms include abdominal pain and bloody diarrhoea. The disease may be well controlled with mesalamine derivatives such as sulfasalazine, olsalazine, or balsalazide, but these drugs may cause serious side effects, including kidney damage and bone marrow suppression, so they must be used cautiously in patients at risk for developing adverse side effects.
• Lymphocytic colitis is a rare type of chronic diarrhoea that may be related to inflammatory bowel disease (IBD). About half of affected individuals have another autoimmune disorder such as irritable bowel syndrome, thyroid problems, or ulcerative colitis.
• Celiac disease is a digestive disorder that results from sensitivity to gluten in the diet. Gluten is a protein found mainly in wheat but also occurs in rye and barley. Symptoms include diarrhoea, anemia, abdominal pain, and weight loss which respond well to a life-long gluten-free diet avoiding all foods containing wheat flour.