Signs and symptoms of spontaneous bacterial peritonitis include fevers, chills, nausea, vomiting, abdominal tenderness, and general malaise. Affected individuals may complain of abdominal pain and worsening ascites. Thirteen percent of patients have no signs or symptoms. Hepatic encephalopathy may be the only manifestation of SBP; in the absence of a clear precipitant for the encephalopathy, all patients should undergo paracentesis, or sampling of the ascites fluid, in order to assess for SBP.

The main manifestations of peritonitis are acute abdominal pain, abdominal tenderness and abdominal guarding, which are exacerbated by moving the peritoneum, e.g., coughing (forced cough may be used as a test), flexing one's hips, or eliciting the Blumberg sign (a.k.a. rebound tenderness, meaning that pressing a hand on the abdomen elicits less pain than releasing the hand abruptly, which will aggravate the pain, as the peritoneum snaps back into place). Rigidity (involuntary contraction of the abdominal muscles) is the most specific exam finding for diagnosing peritonitis (+ likelihood ratio: 3.9). The presence of these signs in a patient is sometimes referred to as peritonism. The localization of these manifestations depends on whether peritonitis is localized (e.g., appendicitis or diverticulitis before perforation), or generalized to the whole abdomen. In either case, pain typically starts as a generalized abdominal pain (with involvement of poorly localizing innervation of the visceral peritoneal layer), and may become localized later (with the involvement of the somatically innervated parietal peritoneal layer). Peritonitis is an example of an acute abdomen.

Spontaneous bacterial peritonitis (SBP) is the development of a bacterial infection in the peritoneum causing peritonitis, despite the absence of an obvious source for the infection. It occurs almost exclusively in people with portal hypertension (increased pressure over the portal vein), usually as a result of cirrhosis of the liver. It can also occur in patients with nephrotic syndrome.

The diagnosis of SBP requires paracentesis (aspiration of fluid with a needle) from the abdominal cavity. If the fluid contains bacteria or large numbers of neutrophil granulocytes (>250 cells/µL) (a type of white blood cells), infection is confirmed and antibiotics are required to avoid complications. In addition to antibiotics, infusions of albumin are usually administered.

Peritonitis is inflammation of the peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs. Symptoms may include severe pain, swelling of the abdomen, fever, or weight loss. One part or the entire abdomen may be tender. Complications may include shock and acute respiratory distress syndrome.

Causes include perforation of the intestinal tract, pancreatitis, pelvic inflammatory disease, stomach ulcer, cirrhosis, or a ruptured appendix. Risk factors include ascites and peritoneal dialysis. Diagnosis is generally based on examination, blood tests, and medical imaging.

Treatment often includes antibiotics, intravenous fluids, pain medication, and surgery. Other measures may include a nasogastric tube or blood transfusion. Without treatment death may occurs within a few days. Approximately 7.5% of people have appendicitis at some point in time. About 20% of people with cirrhosis who are in hospital have peritonitis.

Inflammation can spread to other parts of the gut in patients with typhlitis. The condition can also cause the cecum to become distended and can cut off its blood supply. This and other factors can result in necrosis and perforation of the bowel, which can cause peritonitis and sepsis.

Historically, the mortality rate for typhlitis was as high as 50%, mostly because it is frequently associated with bowel perforation. More recent studies have demonstrated better outcomes with prompt medical management, generally with resolution of symptoms with neutrophil recovery without death

Typhlitis is diagnosed with a radiograph CT scan showing thickening of the cecum and "fat stranding".

Intraabdominal infection (IAI) is a group of infections that occur within the abdominal cavity. They vary from appendicitis to fecal peritonitis. Risk of death despite treatment is often high.

The presentation of acute appendicitis includes abdominal pain, nausea, vomiting, and fever. As the appendix becomes more swollen and inflamed, it begins to irritate the adjoining abdominal wall. This leads to the localization of the pain to the right lower quadrant. This classic migration of pain may not be seen in children under three years. This pain can be elicited through signs and can be severe. Signs include localized findings in the right iliac fossa. The abdominal wall becomes very sensitive to gentle pressure (palpation). There is severe pain on sudden release of deep pressure in the lower abdomen (rebound tenderness). If the appendix is retrocecal (localized behind the cecum), even deep pressure in the right lower quadrant may fail to elicit tenderness (silent appendix). This is because the cecum, distended with gas, protects the inflamed appendix from pressure. Similarly, if the appendix lies entirely within the pelvis, there is usually complete absence of abdominal rigidity. In such cases, a digital rectal examination elicits tenderness in the rectovesical pouch. Coughing causes point tenderness in this area (McBurney's point), historically called Dunphy's sign.

Signs and symptoms usually include a fever greater than , chills, low abdominal pain, and possibly bad smelling vaginal discharge.

Secondary peritonitis and intra-abdominal abscesses including splenic and hepatic abscesses generally occur because of the entry of enteric micro-organisms into the peritoneal cavity through a defect in the wall of the intestine or other viscus as a result of obstruction, infarction or direct trauma. Perforated appendicitis, diverticulitis, inflammatory bowel disease with perforation and gastrointestinal surgery are often associated with polymicrobial infections caused by aerobic and anaerobic bacteria, where the number of isolates can average 12 (two-thirds are generally anaerobes). The most common aerobic and facultative bacteria are "Escherichia coli", "Streptococcus" spp. (including Enterococcus spp.), and the most frequently isolated anaerobic bacteria are the "B. fragilis" group, "Peptostreptococcus" spp., and "Clostridium" spp.

Abdominal infections are characteristically biphasic: an initial stages of generalized peritonitis associated with "Escherichia coli" sepsis, and a later stages, in which intra abdominal abscesses harboring anaerobic bacteria ( including "B. fragilis" group ) emerge.

The clinical manifestations of secondary peritonitis are a reflection of the underlying disease process. Fever, diffuse abdominal pain, nausea and vomiting are common. Physical examination generally show signs of peritoneal inflammation, isuch as rebound tenderness, abdominal wall rigidity and decrease in bowel sounds. These early findings may be followed by signs and symptoms of shock.

Biliary tract infection is usually caused by "E. coli, Klebsiella" and "Enterococcus" spp. Anaerobes (mostly "B. fragilis" group, and rarely "C. perfringens") can be recovered in complicated infections associated with carcinoma, recurrent infection, obstruction, bile tract surgery or manipulation.

Laboratory studies show elevated blood leukocyte count and predominance of polymorphonuclear forms. Radiographs studies may show free air in the peritoneal cavity, evidence of ileus or obstruction and obliteration of the psoas shadow. Diagnostic ultrasound, gallium and CT scanning may detect appendiceal or other intra-abdominal abscesses. Polymicrobial postoperative wound infections can occur.

Treatment of mixed aerobic and anaerobic abdominal infections requires the utilization of antimicrobials effective against both components of the infection as well as surgical correction and drainage of pus. Single and easily accessible abscesses can be drained percutaneously.

Pylephlebitis (also called pyelophlebitis and infective suppurative thrombosis of the portal vein) is an uncommon thrombophlebitis of the portal vein or any of its branches (i.e. a portal vein thrombosis) that is caused by infection. It is usually a complication of intraabdominal sepsis, most often following diverticulitis, perforated appendicitis, or peritonitis. Considered uniformly lethal in the pre-antibiotic era, it still carries a mortality of 10-30%.

It typically presents with fever, rigors, and right upper quadrant abdominal pain, but sometimes abdominal pain may be absent. Liver function test abnormalities are usually present but frank jaundice is uncommon. In the modern era, it is usually diagnosed by CT scans of the abdomen and pelvis. Bacteriology is often polymicrobial and blood cultures are positive in some cases. A significant fraction of people presenting with this condition have an underlying hypercoagulable state.

Treatment is with a prolonged course of broad-spectrum antibiotics, with the addition of anticoagulants if other clots are present outside the portal vein or if fever persists on antibiotic therapy.

It is a cause of portal hypertension and can cause bowel ischemia sometimes leading to bowel infarction.

Anaerobes can be isolated from most types of upper respiratory tract and head and neck and infection and are especially common in chronic ones. These include tonsillar, peritonsillar and retropharyngeal abscesses, chronic otitis media, sinusitis and mastoiditis, eye ocular) infections, all deep neck space infections, parotitis, sialadenitis, thyroiditis, odontogenic infections, and postsurgical and nonsurgical head and neck wounds and abscesses., The predominant organisms are of oropharyngeal flora origin and include AGNB, "Fusobacterium" and Peptostreptococcus spp.

Anaerobes involve almost all dental infections. These include dental abscesses, endodontal pulpitis and periodontal (gingivitis and periodontitis) infections, and perimandibular space infection. Pulpitis can lead to abscess formation and eventually spread to the mandible and other neck spaces. In addition to strict anaerobic bacteria, microaerophilic streptococci and "Streptococcus salivarius" can also be present.

"Fusobacterium" spp. and anaerobic spirochetes are often the cause of acute necrotizing ulcerative gingivitis (or Vincent's angina) which is a distinct form of ulcerative gingivitis.

Deep neck infections that develop as a consequence of oral, dental and pharyngeal infections are generally polymicrobial in nature. These include extension of retropharyngeal cellulitis or abscess, mediastinitis following esophagus perforation, and dental or periodontal abscess.

Tubo-ovarian abscesses (TOA) are one of the late complications of pelvic inflammatory disease (PID) and can be life-threatening if the abscess ruptures and results in sepsis. It consists of an encapsulated or confined 'pocket of pus' with defined boundaries that forms during an infection of a fallopian tube and ovary. These abscesses are found most commonly in reproductive age women and typically result from upper genital tract infection. It is an inflammatory mass involving the fallopian tube, ovary and, occasionally, other adjacent pelvic organs. A TOA can also develop as a complication of a hysterectomy.

Patients typically present with fever, elevated white blood cell count, lower abdominal-pelvic pain, and/or vaginal discharge. Fever and leukocytosis may be absent. TOAs are often polymicrobial with a high percentage of anaerobic bacteria. The cost of treatment is approximately $2,000 per patient, which equals about $1.5 billion annually. Though rare, TOA can occur without a preceding episode of PID or sexual activity.

After childbirth a woman's genital tract has a large bare surface, which is prone to infection. Infection may be limited to the cavity and wall of her uterus, or it may spread beyond to cause septicaemia (blood poisoning) or other illnesses, especially when her resistance has been lowered by a long labour or severe bleeding. Puerperal infection is most common on the raw surface of the interior of the uterus after separation of the placenta (afterbirth); but pathogenic organisms may also affect lacerations of any part of the genital tract. By whatever portal, they can invade the bloodstream and lymph system to cause septicemia, cellulitis (inflammation of connective tissue), and pelvic or generalized peritonitis (inflammation of the abdominal lining). The severity of the illness depends on the virulence of the infecting organism, the resistance of the invaded tissues, and the general health of the woman. Organisms commonly producing this infection are "Streptococcus pyogenes"; staphylococci (inhabitants of the skin and of pimples, carbuncles, and many other pustular eruptions); the anaerobic streptococci, which flourish in devitalized tissues such as may be present after long and injurious labour and unskilled instrumental delivery; "Escherichia coli" and "Clostridium perfringens" (inhabitants of the lower bowel); and "Clostridium tetani".

The signs and symptoms of tubo-ovarian abscess (TOA) are the same as with pelvic inflammatory disease (PID) with the exception that the abscess can be found with magnetic resonance imaging (MRI), sonography and x-ray. It also differs from PID in that it can create symptoms of acute-onset pelvic pain. Typically this disease is found in sexually active women but sexually inexperienced, virginal girls have rarely been found with this infection.

Children: Gastroenteritis, mesenteric adenitis, Meckel's diverticulitis, intussusception, Henoch–Schönlein purpura, lobar pneumonia, urinary tract infection (abdominal pain in the absence of other symptoms can occur in children with UTI), new-onset Crohn's disease or ulcerative colitis, pancreatitis, and abdominal trauma from child abuse; distal intestinal obstruction syndrome in children with cystic fibrosis; typhlitis in children with leukemia.

Women: A pregnancy test is important for all women of childbearing age since an ectopic pregnancy can have signs and symptoms similar to those of appendicitis. Other obstetrical/gynecological causes of similar abdominal pain in women include pelvic inflammatory disease, ovarian torsion, menarche, dysmenorrhea, endometriosis, and Mittelschmerz (the passing of an egg in the ovaries approximately two weeks before menstruation).

Men: testicular torsion

Adults: new-onset Crohn's disease, ulcerative colitis, regional enteritis, cholecystitis, renal colic, perforated peptic ulcer, pancreatitis, rectus sheath hematoma and epiploic appendagitis.

Elderly: diverticulitis, intestinal obstruction, colonic carcinoma, mesenteric ischemia, leaking aortic aneurysm.

The term "" is used to describe a condition mimicking appendicitis. It can be associated with "Yersinia enterocolitica".

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.

Emergency action may be required if severe abdominal pain develops, particularly if it is accompanied by fever, rapid heart rate, tenderness when the abdomen is pressed, bloody diarrhea, frequent diarrhea, or painful bowel movements.

Colonoscopy is contraindicated, as it may rupture the dilated colon resulting in peritonitis and septic shock.

Signs and symptoms may include a sudden pain in the epigastrium to the right of the midline indicating the perforation of a duodenal ulcer. In a gastric ulcer perforation creates a history of burning pain in epigastrium, with flatulence and dyspepsia.

In intestinal perforation, pain starts from the site of perforation and spreads across the abdomen.

Gastrointestinal perforation results in severe abdominal pain intensified by movement, nausea, vomiting and hematemesis. Later symptoms include fever and or chills. In any case, the abdomen becomes rigid with tenderness and rebound tenderness. After some time the abdomen becomes silent and heart sounds can be heard all over. Patient stops passing flatus and motion, abdomen is distended.

The symptoms of esophageal rupture may include sudden onset of chest pain.

Subphrenic abscess is a disease characterized by an accumulation of infected fluid between the diaphragm, liver, and spleen. This abscess develops after surgical operations like splenectomy.

Presents with cough, increased respiratory rate with shallow respiration, diminished or absent breath sounds, hiccups, dullness in percussion, tenderness over the 8th–11th ribs, fever, chills, anorexia and shoulder tip pain on the affected side. Lack of treatment or misdiagnosis could quickly lead to sepsis, septic shock, and death. It is also associated with peritonitis.

Signs and symptoms of enteritis are highly variable and vary based on the specific cause and other factors such as individual variance and stage of disease.

Symptoms may include abdominal pain, cramping, diarrhoea, dehydration, fever, nausea, vomiting and weight loss.

Twenty percent of infants born with meconium peritonitis will have vomiting and dilated bowels on x-rays which necessitates surgery.

Meconium peritonitis is sometimes diagnosed on prenatal ultrasound where it appears as calcifications within the peritoneum.

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

A number of complications may occur from cholecystitis if not detected early or properly treated. Signs of complications include high fever, shock and jaundice. Complications include the following:

- Gangrene

- Gallbladder rupture

- Empyema

- Fistula formation and gallstone ileus

- Rokitansky-Aschoff sinuses

Most people with gallstones do not have symptoms. When a gallstone lodges in the cystic duct, they experience biliary colic. Biliary colic is abdominal pain in the right upper quadrant or epigastric region. It is episodic, occurs after eating greasy or fatty foods, and leads to nausea and/or vomiting. People who suffer from cholecystitis most commonly have symptoms of biliary colic before developing cholecystitis. The pain becomes more severe and constant in cholecystitis. Nausea is common and vomiting occurs in 75% of people with cholecystitis. In addition to abdominal pain, right shoulder pain can be present.

On physical examination, fever is common. A gallbladder with cholecystitis is almost always tender to touch. Because of the inflammation, its size can be felt from the outside of the body in 25–50% of people with cholecystitis. Pain with deep inspiration leading to termination of the breath while pressing on the right upper quadrant of the abdomen usually causes pain (Murphy's sign). Murphy's sign is sensitive, but not specific for cholecystitis. Yellowing of the skin (jaundice) may occur but is often mild. Severe jaundice suggests another cause of symptoms such as choledocholithiasis. People who are old, have diabetes, chronic illness, or who are immunocompromised may have vague symptoms that may not include fever or localized tenderness.