The causes of diverticulitis are poorly understood, with approximately 40 percent due to genes and 60 percent due to environmental factors. Conditions that increase the risk of developing diverticulitis include arterial hypertension and immunosuppression. Obesity is another risk factor.

The condition is diagnosed most often in infancy and early childhood. It strikes about 2,000 infants (one in every 1,900) in the United States in the first year of life. Its incidence begins to rise at about one to five months of life, peaks at four to nine months of age, and then gradually declines at around 18 months.

Intussusception occurs more frequently in boys than in girls, with a ratio of approximately 3:1.

In adults, intussusception represents the cause of approximately 1% of bowel obstructions and is frequently associated with neoplasm, malignant or otherwise.

Causes of intussusception are not clearly established or understood. About 90% of cases of intussusception in children arise from an unknown cause. They can include infections, anatomical factors, and altered motility.

- Meckel's diverticulum

- Polyp

- Duplication

- Appendix

- Hyperplasia of Peyer's patches

- Idiopathic

An earlier version of the rotavirus vaccine that is no longer used was linked to intussusception, but the current versions are not clearly linked. Due to a potential risk, they are thus not recommended in babies who have had intussusception.

It is unclear what role dietary fibre plays in diverticulitis. It is often stated that a diet low in fibre is a risk factor; however, the evidence to support this is unclear. There is no evidence to suggest that the avoidance of nuts and seeds prevents the progression of diverticulosis to an acute case of diverticulitis. It appears in fact that a higher intake of nuts and corn could help to avoid diverticulitis in adult males.

Zenker's diverticulum mainly affects older adults. It has an incidence of 2 per 100,000 per year in the UK, but there is significant geographical variation around the world.

A Killian–Jamieson diverticulum is an outpouching of the esophagus just below the upper esophageal sphincter.

The physicians that first discovered the diverticulum were Gustav Killian and James Jamieson. Diverticula are seldom larger than 1.5 cm, and are less frequent than the similar Zenker's diverticula. As opposed to a Zenker's, which is typically a posterior and inferior outpouching from the esophagus, a Killian–Jamieson diverticulum is typically an anterolateral outpouching at the level of the C5-C6 vertebral bodies, due to a congenital weakness in the cervical esophagus just below the cricopharyngeal muscle. It is usually smaller in size than a Zenker's diverticulum, and typically asymptomatic. Although congenital, it is more commonly seen in elderly patients.

Meckel's diverticulum occurs in about 2% of the population. Prevalence in males is 3–5 times higher than in females. Only 2% of cases are symptomatic, which usually presents among children at the age of 2.

Most cases of Meckel's diverticulum are diagnosed when complications manifest or incidentally in unrelated conditions such as laparotomy, laparoscopy or contrast study of the small intestine. Classic presentation in adults includes intestinal obstruction and inflammation of the diverticulum (diverticulitis). Painless rectal bleeding most commonly occurs in toddlers.

Inflammation in the ileal diverticulum has symptoms that mimic appendicitis, therefore its diagnosis is of clinical importance. Detailed knowledge of the pathophysiological properties is essential in dealing with the life-threatening complications of Meckel's diverticulum.

In simple words, when there is excessive pressure within the lower pharynx, the weakest portion of the pharyngeal wall balloons out, forming a diverticulum which may reach several centimetres in diameter.

More precisely, while traction and pulsion mechanisms have long been deemed the main factors promoting development of a Zenker's diverticulum, current consensus considers occlusive mechanisms to be most important: uncoordinated swallowing, impaired relaxation and spasm of the cricopharyngeus muscle lead to an increase in pressure within the distal pharynx, so that its wall herniates through the point of least resistance (known as Killian's triangle, located superior to the cricopharyngeus muscle and inferior to the Thyropharyngeus muscle. Thyropharyngeus and Laryngopharyngeus are the superior and inferior parts of inferior constrictor muscle of pharynx respectively). The result is an outpouching of the posterior pharyngeal wall, just above the esophagus.

While it may be asymptomatic, Zenker diverticulum can present with the following symptoms:

- Dysphagia (difficulty swallowing), and sense of a lump in the throat

- Food might get trapped in the outpouching, leading to:

- Regurgitation, reappearance of ingested food in the mouth

- Cough, due to food regurgitated into the airway

- Halitosis, smelly breath, as stagnant food is digested by microorganisms

- Infection

It rarely, if ever, causes any pain.

Cervical webs are seen associated in 50% of patients with this condition.

Rarer forms of cervical esophageal diverticula are the Killian's diverticulum and the Laimer's diverticulum. Killian's diverticulum is formed in the Killian-Jamiseon triangle (located inferior to the cricopharyngeus on both sides of this muscle's insertion into the cricoid cartilage). Laimer's diverticulum is formed in Laimer's triangle (located inferior to the cricopharyngeus in the posterior midline above the confluence of the longitudinal layer of esophageal muscle). Laimer's triangle is covered only by the circular layer of esophageal muscle.

Urethral diverticulum can occur in men, and can cause complications including kidney stones and urinary tract infections.

The prevalence of diverticulosis progressively increases with age. Approximately 50% of people over the age of 60 and 70% of people over the age of 80 have diverticulosis. This disease is common in the U.S., Britain, Australia, Canada, and is uncommon in Asia and Africa. Large-mouth diverticula are associated with scleroderma. Diverticular disease is more common in collagen disorders such as Ehlers Danlos Syndrome.

The U.S. National Institutes of Health notes that, although the low-fiber theory of the cause of diverticulosis is the leading theory, it has not yet been proven.

Diverticula may occur in one of the three areas of the esophagus - the pharyngoesophageal, the midesophageal area or the epiphrenic area of esophagus. Zenker's diverticulum is found three times more frequently in men than in women. It occurs posteriorly through the cricopharyngeal muscle in the midline of the neck. Usually seen in people older than 60 years of age.

- Gastric diverticula - "Although usually asymptomatic, they may cause vague epigastric pain. These lesions may be confused radiologically for gastric ulcers or cancers. Endoscopically, they may be confused for paraesophageal hernias."

- Killian-Jamieson diverticulum

- Meckel's diverticulum: a persistent portion of the omphalomesenteric duct present in 2% of the population

- Rokitansky-Aschoff sinuses: in the gallbladder due to chronic cholecystitis

- Traction esophageal diverticulum: due to scarring from mediastinal or pulmonary tuberculosis

- Urethral diverticulum: congenital in males, post-infectious in females

- Zenker's diverticulum: a diverticulum of the mucosa of the pharynx affecting adults

Most of these pathological types of diverticulum are capable of harboring an enterolith. If the enterolith stays in place, it may cause no problems, but a large enterolith expelled from a diverticulum into the lumen can cause obstruction.

The incidence of urethral diverticulum has been increasing in the 2000s, likely due to increasing diagnosis and detection of the condition. It is estimated to be present in as low as 0.02% of all women and as high as 6% of all women, and 40% of women with lower urinary tract symptoms. Most symptomatic urethral diverticula are present in women from 30–60 years old.

84% of periurethral masses are due to urethral diverticula.

A diverticulum (plural: "diverticula") is the medical or biological term for an outpouching of a hollow (or a fluid-filled) structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false.

In medicine, the term usually implies the structure is not normally present. However, in the embryonic stage, some normal structures begin development as a diverticulum arising from another structure.

The lifetime risk for a person with Meckel's diverticulum to develop certain complications is about 4–6%. Gastrointestinal bleeding, peritonitis or intestinal obstruction may occur in 15–30% of symptomatic patients (Table 1). Only 6.4% of all complications requires surgical treatment; and untreated Meckel's diverticulum has a mortality rate of 2.5–15%.

Table 1 – Complications of Meckel's Diverticulum:

Equine enteroliths are found by walking pastures or turning over manure compost piles to find small enteroliths, during necroscopy, and increasingly, during surgery for colic. Therefore, the incidence of asymptomatic enteroliths is unknown.

Equine enteroliths typically are smoothly spherical or tetrahedral, consist mostly of the mineral struvite (ammonium magnesium phosphate), and have concentric rings of mineral precipitated around a nidus.

Enteroliths in horses were reported widely in the 19th century, infrequently in the early 20th century, and now increasingly. They have also been reported in zebras: five in a zoo in California and one in a zoo in Wisconsin. Struvite enteroliths are associated with elevated pH and mineral concentrations in the lumen. In California, struvite enteroliths are associated also with a high proportion of alfalfa in the feed and less access to grass pasture. This association has been attributed to the cultivation of alfalfa on serpentine soils, resulting in high concentrations of magnesium in the alfalfa.

An enterolith is a mineral concretion or calculus formed anywhere in the gastrointestinal system. Enteroliths are uncommon and usually incidental findings but, once found, they require at a minimum watchful waiting. If there is evidence of complications, they must be removed.

An enterolith may form around a "nidus", a small foreign object such as a seed, pebble, or piece of twine, that serves as an irritant. In this respect, an enterolith forms by a process similar to the creation of a pearl.

An enterolith is not to be confused with a gastrolith, which helps digestion.

FI is thought to be very common, but much under-reported due to embarrassment. One study reported a prevalence of 2.2% in the general population. It affects people of all ages, but is more common in older adults (but it should not be considered a normal part of aging). Females are more likely to develop it than males (63% of those with FI over 30 may be female). In 2014, the National Center for Health Statistics reported that one out of every six seniors in the U.S. who lived in their own home or apartment had FI. Men and women were equally affected. 45–50% of people with FI have severe physical and/or mental disabilities.

Risk factors include age, female gender, urinary incontinence, history of vaginal delivery (non-Caesarean section childbirth), obesity, prior anorectal surgery, poor general health and physical limitations. Combined urinary and fecal incontinence is sometimes termed double incontinence, and it is more likely to be present in those with urinary incontinence.

Traditionally, FI was thought to be an insignificant complication of surgery, but it is now known that a variety of different procedures are associated with this possible complication, and sometimes at high levels. Examples are midline internal sphincterotomy (8% risk), lateral internal sphincterotomy, fistulectomy, fistulotomy (18-52%), hemorrhoidectomy (33%), ileo-anal reservoir reconstruction, lower anterior resection, total abdominal colectomy, ureterosigmoidostomy, and anal dilation (Lord's procedure, 0-50%). Some authors consider obstetric trauma to be the most common cause.

This may occur when there is a large mass of feces in the rectum (fecal loading), which may become hardened (fecal impaction). Liquid stool elements are able to pass around the obstruction, leading to incontinence. Megarectum (enlarged rectal volume) and rectal hyposensitivity are associated with overflow incontinence. Hospitalized patients and care home residents may develop FI via this mechanism, possibly a result of lack of mobility, reduced alertness, constipating effect of medication and/or dehydration.

Many people are managed through day surgery centers, and are able to return to work within a week or two, while intense activities are prohibited for a longer period. People who have their hernias repaired with mesh often recover within a month, though pain can last longer. Surgical complications include pain that lasts more than three months, surgical site infections, nerve and blood vessel injuries, injury to nearby organs, and hernia recurrence. Pain that lasts more than three months occurs in about 10% of people following hernia repair.

Medication (to prevent spasms) or Sphincterotomy (surgical procedure to cut the muscle) are the standard treatments for sphincter of Oddi dysfunction. One or the other may be better based on the classification of the condition.

Diseases causing inflammation in the GI tract can lead to blood in the stool. Inflammation can occur anywhere along the GI tract in Crohn's disease, or in the colon if a person has ulcerative colitis.

- Crohns disease

- Ulcerative colitis

This list of diagnoses include diseases in which the wall of the bowel is compromised by disease.

- Peptic ulcer disease—divided into either duodenal or gastric ulcers, most common common causes include:

- Non steroidal anti-inflammatory drug (NSAID)—the use of these medications results in a structural change in the wall of the gut, namely ulcers, and potential blood in the stool.

- "H. pylori" infection—this bacterial infection can erode the wall of the stomach or duodenum, leading to a structural change in the stomach wall and bleeding in the stool.

- Chronic disease

- Diverticulitis and diverticulosis result from an out pouching of the colonic mucosa, or gut wall, leading to a break down of weak gut wall and an increased susceptibility to infection due to the bacteria in the GI tract, thus the potential for vascular compromise, the collection of bacteria in the area of perforation (abscess), the abnormal formation of communication between another part of the hollow GI tract (fistula), or blockage of the bowel (obstruction).

- Meckel's diverticulum is a congenital remnant of the omphalo-mesenteric duct that connected the fetal yolk sac to the intestines which is normal closed off and destroyed during the process of development. If a portion, or all of this duct remains a diverticulum or fistula can result, leading to the potential for a source of bleeding.

Two mechanisms are involved in the development of sphincter of Oddi dysfunction, either or both of which may be contributory to the condition: stenosis, or narrowing of the sphincter of Oddi (also termed papillary stenosis), and dyskinesia, or alteration in the function of the sphincter of Oddi (also termed biliary dyskinesia). Individuals with stenosis of the sphincter of Oddi typically have an elevated baseline pressure of the sphincter of Oddi, due to an anatomical problem that leads to narrowing of the sphincter, such as recurrent passage of gallstones through the ampulla of Vater, trauma to the sphincter from procedures such as endoscopic retrograde cholangiopancreatography or biliary surgery, or infections of the common bile duct. In contrast, dyskinesia of the sphincter of Oddi is a purely functional disorder, wherein there is intermittent obstruction of the bile duct due to inappropriate spasms. The reasons for dyskinesia of the sphincter of Oddi are not completely understood, but believed to be due to alteration in local gut hormones and peptides, such as cholecystokinin, which act on the sphincter or to altered neuronal control of the sphincter.

Choledochal cysts are treated by surgical excision of the cyst with the formation of a roux-en-Y anastomosis hepaticojujenostomy/ choledochojujenostomy to the biliary duct.

Future complications include cholangitis and a 2% risk of malignancy, which may develop in any part of the biliary tree. A recent article published in Journal of Surgery suggested that choledochal cysts could also be treated with single-incision laparoscopic hepaticojejunostomy with comparable results and less scarring. In cases of saccular type of cyst, excision and placement of T-shaped tube is done.

Currently, there is no accepted indication for fetal intervention in the management of prenatally suspected choledochal cysts.