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.

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.

The predisposition to diverticulosis for specific individuals is likely explained by a genetic component, a theory which is supported by studies examining the rates of diverticulosis among twins. The heritability of diverticulosis is estimated to be approximately 40%.

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.

The prognosis for non-ischemic cases of SBO is good with mortality rates of 3–5%, while prognosis for SBO with ischemia is fair with mortality rates as high as 30%.

Cases of SBO related to cancer are more complicated and require additional intervention to address the malignancy, recurrence, and metastasis, and thus are associated with poorer prognosis.

All cases of abdominal surgical intervention are associated with increased risk of future small-bowel obstructions. Statistics from U.S. healthcare report 18.1% re-admittance rate within 30 days for patients who undergo SBO surgery. More than 90% of patients also form adhesions after major abdominal surgery.

Common consequences of these adhesions include small-bowel obstruction, chronic abdominal pain, pelvic pain, and infertility.

Fetal and neonatal bowel obstructions are often caused by an intestinal atresia, where there is a narrowing or absence of a part of the intestine. These atresias are often discovered before birth via an ultrasound, and treated with using laparotomy after birth. If the area affected is small, then the surgeon may be able to remove the damaged portion and join the intestine back together. In instances where the narrowing is longer, or the area is damaged and cannot be used for a period of time, a temporary stoma may be placed.

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.

Treatment is surgical, potentially with a laparoscopic resection. In patients with bleeding, strangulation of bowel, bowel perforation or bowel obstruction, treatment involves surgical resection of both the Meckel's diverticulum itself along with the adjacent bowel segment, and this procedure is called a "small bowel resection". In patients without any of the aforementioned complications, treatment involves surgical resection of the Meckel's diverticulum only, and this procedure is called a simple diverticulectomy.

With regards to asymptomatic Meckel's diverticulum, some recommend that a search for Meckel's diverticulum should be conducted in every case of appendectomy/laparotomy done for acute abdomen, and if found, Meckel's diverticulectomy or resection should be performed to avoid secondary complications arising from it.

Fecal impaction and attempts at removal can have severe and even lethal effects, such as the rupture of the colon wall by catheter or an acute angle of the fecaloma (stercoral perforation), followed by septicemia. A small fecalith is one cause of both appendicitis and acute diverticulitis. It may also lead to stercoral perforation, a condition characterized by bowel perforation due to pressure necrosis from a fecal mass or fecaloma.

Laparotomy for other forms of volvulus, especially anal volvulus.

Distal or sigmoid, fecalomas can often be disimpacted digitally or by a catheter which carries a flow of disimpaction fluid (water or other solvent or lubricant). Surgical intervention in the form of sigmoid colectomy or proctocolectomy and ileostomy may be required only when all conservative measures of evacuation fail.

In a cecal volvulus, the cecum may be returned to a normal position and sutured in place, a procedure known as cecopexy. If identified early, before presumed intestinal wall ischemia has resulted in tissue breakdown and necrosis, the cecal volvulus can be detorsed laparoscopically.

Many causes exist including:

- diverticulitis : most common ~ 60%

- colorectal cancer (CRC) : ~ 20%

- Crohn's disease : ~ 10%

- radiotherapy

- appendicitis

- trauma

If fecal matter passes through the fistula into the bladder, the existence of the fistula may be revealed by pneumaturia or fecaluria.

Diverticular disease is when problems occur due to diverticulosis, a condition defined by the presence of pouches in the wall of the large intestine (diverticula). This includes diverticula becoming inflamed (diverticulitis) or bleeding. Colonic perforation due to diverticular disease may be classified using the Hinchey Classification.

The differential diagnoses of acute abdomen include but are not limited to:

1. Acute appendicitis

2. Acute peptic ulcer and its complications

3. Acute cholecystitis

4. Acute pancreatitis

5. Acute intestinal ischemia (see section below)

6. Acute diverticulitis

7. Ectopic pregnancy with tubal rupture

8. Ovarian torsion

9. Acute peritonitis (including hollow viscus perforation)

10. Acute ureteric colic

11. Bowel volvulus

12. Bowel obstruction

13. Acute pyelonephritis

14. Adrenal crisis

15. Biliary colic

16. Abdominal aortic aneurysm

17. Familial Mediterranean fever

18. Hemoperitoneum

19. Ruptured spleen

20. Kidney stone

21. Sickle cell anaemia

Underlying causes include gastric ulcers, duodenal ulcers, appendicitis, gastrointestinal cancer, diverticulitis, inflammatory bowel disease, superior mesenteric artery syndrome, trauma and ascariasis. Typhoid fever, non-steroidal anti-inflammatory drugs, ingestion of corrosives may also be responsible.

Acute abdomen is occasionally used synonymously with peritonitis. While this is not entirely incorrect, peritonitis is the more specific term, referring to inflammation of the peritoneum. It manifests on physical examination as rebound tenderness, or pain upon "removal" of pressure more than on "application" of pressure to the abdomen. Peritonitis may result from several of the above diseases, notably appendicitis and pancreatitis. While rebound tenderness is commonly associated with peritonitis, the most specific finding is rigidity.

Gastrointestinal perforation, also known as ruptured bowel, is a hole in the wall of part of the gastrointestinal tract. The gastrointestinal tract includes the esophagus, stomach, small intestine, and large intestine. Symptoms include severe abdominal pain and tenderness. When the hole is in the stomach or early part of the small intestine the onset of pain is typically sudden while with a hole in the large intestine onset may be more gradual. The pain is usually constant in nature. Sepsis, with an increased heart rate, increased breathing rate, fever, and confusion may occur.

The cause can include trauma such as from a knife wound, eating a sharp object, or a medical procedure such as colonoscopy, bowel obstruction such as from a volvulus, colon cancer, or diverticulitis, stomach ulcers, ischemic bowel, and a number of infections including "C. difficile". A hole allows intestinal contents to enter the abdominal cavity. The entry of bacteria results in a condition known as peritonitis or in the formation of an abscess. A hole in the stomach can also lead to a chemical peritonitis due to gastric acid. A CT scan is typically the preferred method of diagnosis; however, free air from a perforation can often be seen on plain X-ray.

Perforation anywhere along the gastrointestinal tract typically requires emergency surgery in the form of an exploratory laparotomy. This is usually carried out along with intravenous fluids and antibiotics. A number of different antibiotics may be used such as piperacillin/tazobactam or the combination of ciprofloxacin and metronidazole. Occasionally the hole can be sewn closed while other times a bowel resection is required. Even with maximum treatment the risk of death can be as high as 50%. A hole from a stomach ulcer occurs in about 1 per 10,000 people per year, while one from diverticulitis occurs in about 0.4 per 10,000 people per year.

Appendicitis is most common between the ages of 5 and 40; the median age is 28. It tends to affect males, those in lower income groups and, for unknown reasons, people living in rural areas. In 2013 it resulted in 72,000 deaths globally down from 88,000 in 1990.

In the United States, there were nearly 293,000 hospitalizations involving appendicitis in 2010. Appendicitis is one of the most frequent diagnoses for emergency department visits resulting in hospitalization among children ages 5–17 years in the United States.

Most people with appendicitis recover easily after surgical treatment, but complications can occur if treatment is delayed or if peritonitis occurs. Recovery time depends on age, condition, complications, and other circumstances, including the amount of alcohol consumption, but usually is between 10 and 28 days. For young children (around 10 years old), the recovery takes three weeks.

The possibility of peritonitis is the reason why acute appendicitis warrants speedy evaluation and treatment. People with suspected appendicitis may have to undergo a medical evacuation. Appendectomies have occasionally been performed in emergency conditions (i.e., not in a proper hospital), when a timely medical evacuation was impossible.

Typical acute appendicitis responds quickly to appendectomy and occasionally will resolve spontaneously. If appendicitis resolves spontaneously, it remains controversial whether an elective interval appendectomy should be performed to prevent a recurrent episode of appendicitis. Atypical appendicitis (associated with suppurative appendicitis) is more difficult to diagnose and is more apt to be complicated even when operated early. In either condition, prompt diagnosis and appendectomy yield the best results with full recovery in two to four weeks usually. Mortality and severe complications are unusual but do occur, especially if peritonitis persists and is untreated.

Another entity known as appendicular lump is talked about. It happens when the appendix is not removed early during infection and omentum and intestine adhere to it, forming a palpable lump. During this period, surgery is risky unless there is pus formation evident by fever and toxicity or by USG. Medical management treats the condition.

An unusual complication of an appendectomy is "stump appendicitis": inflammation occurs in the remnant appendiceal stump left after a prior incomplete appendectomy. Stump appendicitis can occur months to years after initial appendectomy and can be identified with imaging modalities like ultrasound.

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

Diseases of the hepatobiliary system affect the biliary tract (also known as the "biliary tree"), which secretes bile in order to aid digestion of fats. Diseases of the gallbladder and bile ducts are commonly diet-related, and may include the formation of gallstones that impact in the gallbladder (cholecystolithiasis) or in the common bile duct (choledocholithiasis).

Gallstones are a common cause of inflammation of the gallbladder, called cholecystitis. Inflammation of the biliary duct is called cholangitis, which may be associated with autoimmune disease, such as primary sclerosing cholangitis, or a result of bacterial infection, such as ascending cholangitis.

Disease of the biliary tree may cause pain in the upper right abdomen, particularly when pressed. Disease might be investigated using ultrasound or ERCP, and might be treated with drugs such as antibiotics or UDCA, or by the surgical removal of the gallbladder.

Bowel infarction results from restricted blood supply to the bowel, most often due to bowel obstruction or occlusion of one of the mesenteric arteries.

Bowel obstruction is most often caused by intestinal adhesions, which frequently form after abdominal surgeries, or by chronic infections such as diverticulitis, hepatitis, and inflammatory bowel disease. The condition is difficult to diagnose properly, as the symptoms may resemble those of other bowel disorders.

Patients who have undergone extensive resection of the small bowel may develop malabsorption, indicating the need for dietary supplements.

Volvulus is a rare but life-threatening cause of bowel infarction which requires immediate medical attention. Central abdominal pain which is resistant to narcotic analgesia may be an indication of bowel infarction.

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.