Known symptoms of duodenitis include:

- Abdominal pain

- vomiting

- nausea

- discomfort in stomach

Many people with gastritis experience no symptoms at all. However, upper central abdominal pain is the most common symptom; the pain may be dull, vague, burning, aching, gnawing, sore, or sharp. Pain is usually located in the upper central portion of the abdomen, but it may occur anywhere from the upper left portion of the abdomen around to the back.

Other signs and symptoms may include the following:

- Nausea

- Vomiting (if present, may be clear, green or yellow, blood-streaked, or completely bloody, depending on the severity of the stomach inflammation)

- Belching (if present, usually does not relieve the pain much)

- Bloating

- Early satiety

- Loss of appetite

- Unexplained weight loss

Diagnosis is generally made by endoscopy with biopsy to evaluate histology. Review of symptoms and associated conditions is important.

Common causes include "Helicobacter pylori" and NSAIDs. Less common causes include alcohol, cocaine, severe illness and Crohn disease, among others.

Cytomegalovirus colitis, also known as CMV colitis, is an inflammation of the colon

Causes

The infection is spread by saliva, urine, respiratory droplets, sexual contact, and blood transfusions. Most people are exposed to the virus in their lifetime, but it usually produces mild or no symptoms in healthy people.

However, serious CMV infections can occur in people with weakened immune systems. This includes patients receiving chemotherapy for cancer and patients on immune-suppressing medicines following an organ transplant.

In rare instances, more severe CMV infection involving the GI tract has been reported in people with a healthy immune system.

Researchers have published conflicting reports concerning whether "Blastocystis" causes symptoms in humans, with one of the earliest reports in 1916. The incidence of reports associated with symptoms began to increase in 1984, with physicians from Saudi Arabia reporting symptoms in humans and US physicians reporting symptoms in individuals with travel to less developed countries. A lively debate ensued in the early 1990s, with some physicians objecting to publication of reports that "Blastocystis" caused disease. Some researchers believe the debate has been resolved by finding of multiple species of "Blastocystis" that can infect humans, with some causing symptoms and others being harmless (see Genetics and Symptoms).

A few of most commonly reported symptoms are:

- abdominal pain

- itching, usually anal itching

- constipation

- diarrhea

- watery or loose stools

- weight loss

- fatigue

- flatulence

Some less commonly reported symptoms include:

- Skin rash

- Headache, depression

- Arthritic symptoms and joint pain

- Intestinal inflammation

Signs and symptoms of a peptic ulcer can include one or more of the following:

- abdominal pain, classically epigastric strongly correlated to mealtimes. In case of duodenal ulcers the pain appears about three hours after taking a meal;

- bloating and abdominal fullness;

- waterbrash (rush of saliva after an episode of regurgitation to dilute the acid in esophagus - although this is more associated with gastroesophageal reflux disease);

- nausea and copious vomiting;

- loss of appetite and weight loss;

- hematemesis (vomiting of blood); this can occur due to bleeding directly from a gastric ulcer, or from damage to the esophagus from severe/continuing vomiting.

- melena (tarry, foul-smelling feces due to presence of oxidized iron from hemoglobin);

- rarely, an ulcer can lead to a gastric or duodenal perforation, which leads to acute peritonitis, extreme, stabbing pain, and requires immediate surgery.

A history of heartburn, gastroesophageal reflux disease (GERD) and use of certain forms of medication can raise the suspicion for peptic ulcer. Medicines associated with peptic ulcer include NSAIDs (non-steroid anti-inflammatory drugs) that inhibit cyclooxygenase, and most glucocorticoids (e.g. dexamethasone and prednisolone).

In people over the age of 45 with more than two weeks of the above symptoms, the odds for peptic ulceration are high enough to warrant rapid investigation by esophagogastroduodenoscopy.

The timing of the symptoms in relation to the meal may differentiate between gastric and duodenal ulcers: A gastric ulcer would give epigastric pain during the meal, as gastric acid production is increased as food enters the stomach. Symptoms of duodenal ulcers would initially be relieved by a meal, as the pyloric sphincter closes to concentrate the stomach contents, therefore acid is not reaching the duodenum. Duodenal ulcer pain would manifest mostly 2–3 hours after the meal, when the stomach begins to release digested food and acid into the duodenum.

Also, the symptoms of peptic ulcers may vary with the location of the ulcer and the person's age. Furthermore, typical ulcers tend to heal and recur and as a result the pain may occur for few days and weeks and then wane or disappear. Usually, children and the elderly do not develop any symptoms unless complications have arisen.

Burning or gnawing feeling in the stomach area lasting between 30 minutes and 3 hours commonly accompanies ulcers. This pain can be misinterpreted as hunger, indigestion or heartburn. Pain is usually caused by the ulcer but it may be aggravated by the stomach acid when it comes into contact with the ulcerated area. The pain caused by peptic ulcers can be felt anywhere from the navel up to the sternum, it may last from few minutes to several hours and it may be worse when the stomach is empty. Also, sometimes the pain may flare at night and it can commonly be temporarily relieved by eating foods that buffer stomach acid or by taking anti-acid medication. However, peptic ulcer disease symptoms may be different for every sufferer.

A gastric peptic ulcer is a mucosal perforation which penetrates the muscularis mucosae and lamina propria, usually produced by acid-pepsin aggression. Ulcer margins are perpendicular and present chronic gastritis. During the active phase, the base of the ulcer shows 4 zones: fibrinoid necrosis, inflammatory exudate, granulation tissue and fibrous tissue. The fibrous base of the ulcer may contain vessels with thickened wall or with thrombosis.

The diagnosis of CMV colitis is based on serology, CMV antigen testing and colonscopy with biopsy.

Clinical suspicion should be aroused in the setting of immunocompromised patient but it is much rarer in immunocompetent patient.

Although it is known that CMV colitis is almost always caused by reactivation of latent CMV infection in immunocompromised patients, new infection of CMV or reinfection of different strain of CMV can cause colitis in immunocompetent hosts.

Because asymptomatic CMV viremia and viruria is common and about 1/3 of symptomatic CMV infection is caused by reinfection of different strain of CMV, the diagnosis of CMV colitis needs more direct causality. It is practically achieved by colonoscopy or sigmoidoscopy tissue sampling and pathological evidence of CMV infection under microscope. Positive CMV IgG doesn't necessarily mean that it is reactivation of latent infection because of the possibility of reinfection of different strain.

Anoscopy can be used to diagnose the majority of cases of proctocolitis.

Blastocystosis refers to a medical condition caused by infection with "Blastocystis". "Blastocystis" is a protozoal, single-celled parasite that inhabits the gastrointestinal tracts of humans and other animals. Many different types of "Blastocystis" exist, and they can infect humans, farm animals, birds, rodents, amphibians, reptiles, fish, and even cockroaches. Blastocystosis has been found to be a possible risk factor for development of IBS (Irritable Bowel Syndrome).

In most cases AIT is characterized by onset of pain, firmness, tenderness, redness or swelling in the anterior aspect of the neck. Patients will also present with a sudden fever, dysphagia and dysphonia. Symptoms may be present from 1 to 180 days, with most symptoms lasting an average of about 18 days. The main issue associated with the diagnosis of AIT is differentiating it from other more commonly seen forms of thyroid conditions. Pain, fever and swelling are often much more severe and continue to get worse in patients who have AIT compared to those with other thyroid conditions. Patients who are suspected of having AIT often undergo tests to detect for elevated levels of white blood cells as well as an ultrasound to reveal unilobular swelling. Depending on the age and immune status of the patient more invasive procedures may be performed such as fine needle aspiration of the neck mass to facilitate a diagnosis.

In cases where the infection is thought to be associated with a sinus fistula it is often necessary to confirm the presence of the fistula through surgery or laryngoscopic examination. While invasive procedures can often tell definitively whether or not a fistula is present, new studies are working on the use of computed tomography as a useful method to visualize and detect the presence of a sinus fistula.

The most commonly reported symptoms in conjunction with infection with "D. fragilis" include abdominal pain (69%) and diarrhea (61%). Diarrhea may be intermittent and may not be present in all cases. It is often chronic, lasting over two weeks. The degree of symptoms may vary from asymptomatic to severe, and can include weight loss, vomiting, fever, and involvement of other digestive organs.

Symptoms may be more severe in children. Additional symptoms reported have included:

1. Weight loss

2. Fatigue

3. Nausea and vomiting

4. Fever

5. Urticaria (skin rash)

6. Pruritus (itchiness)

7. Biliary infection

Pancreatic diseases that affect digestion refers to disorders affecting the exocrine pancreas, which is a part of the pancreas involved in digestion.

One of the most common conditions of the exocrine pancreas is acute pancreatitis, which in the majority of cases relates to gallstones that have impacted in the pancreatic part of the biliary tree, or due to acute or chronic alcohol abuse or as a side-effect of ERCP. Other forms of pancreatitis include chronic and hereditary forms. Chronic pancreatitis may predispose to pancreatic cancer and is strongly linked to alcohol use. Other rarer diseases affecting the pancreas may include pancreatic pseudocysts, exocrine pancreatic insufficiency, and pancreatic fistulas.

Pancreatic disease may present with or without symptoms. When symptoms occur, such as in acute pancreatitis, a person may suffer from acute-onset, severe mid-abdominal pain, nausea and vomiting. In severe cases, pancreatitis may lead to rapid blood loss and systemic inflammatory response syndrome. When the pancreas is unable to secrete digestive enzymes, such as with a pancreatic cancer occluding the pancreatic duct, result in jaundice. Pancreatic disease might be investigated using abdominal x-rays, MRCP or ERCP, CT scans, and through blood tests such as measurement of the amylase and lipase enzymes.

Proctocolitis has many possible causes. Common infectious causes of proctocolitis include Chlamydia trachomatis, LGV (Lymphogranuloma venereum), Neisseria gonorrhoeae, HSV, and Helicobacter species. It can also be idiopathic (see colitis), vascular (as in ischemic colitis), or autoimmune (as in inflammatory bowel disease).

An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut flora), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.

Diseases that affect the large intestine may affect it in whole or in part. Appendicitis is one such disease, caused by inflammation of the appendix. Generalised inflammation of the large intestine is referred to as colitis, which when caused be the bacteria "Clostridium difficile" is referred to as pseudomembranous colitis. Diverticulitis is a common cause of abdominal pain resulting from outpouchings that particularly affects the colon. Functional colonic diseases refer to disorders without a known cause, and include irritable bowel syndrome and intestinal pseudoobstruction. Constipation may result from lifestyle factors, impaction of a rigid stool in the rectum, or in neonates, Hirschprung's disease.

Diseases affecting the large intestine may cause blood to be passed with stool, may cause constipation, or may result in abdominal pain or a fever. Tests that specifically examine the function of the large intestine include barium swallows, abdominal x-rays, and colonoscopy.

The prodromal symptoms are fever, headache, and myalgia, which can be severe, lasting as long as 24 hours. After 1–5 days, typically, these are followed by diarrhea (as many as 10 watery, frequently bloody, bowel movements per day) or dysentery, cramps, abdominal pain, and fever as high as 40 °C (104 °F). In most people, the illness lasts for 2–10 days. It is classified as invasive/inflammatory diarrhea, also described as bloody diarrhea or dysentery.

There are other diseases showing similar symptoms. For instance, abdominal pain and tenderness may be very localized, mimicking acute appendicitis. Furthermore, "Helicobacter pylori" is closely related to Campylobacter and causes peptic ulcer disease.

Complications include toxic megacolon, dehydration and sepsis. Such complications generally occur in young children (< 1 year of age) and immunocompromised people. A chronic course of the disease is possible; this disease process is likely to develop without a distinct acute phase. Chronic campylobacteriosis features a long period of sub-febrile temperature and asthenia; eye damage, arthritis, endocarditis may develop if infection is untreated.

Occasional deaths occur in young, previously healthy individuals because of blood volume depletion (due to dehydration), and in persons who are elderly or immunocompromised.

Some individuals (1–2 in 100,000 cases) develop Guillain–Barré syndrome, in which the nerves that join the spinal cord and brain to the rest of the body are damaged, sometimes permanently. This occurs only with infection of "C. jejuni" and "C. upsaliensis".

A subclinical infection (sometimes called a preinfection) is an infection that, being , is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals. An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

Acute infectious thyroiditis (AIT) also known as suppurative thyroiditis, microbial inflammatory thyroiditis, pyrogenic thyroiditis and bacterial thyroiditis.

The thyroid is normally very resistant to infection. Due to a relatively high amount of iodine in the tissue, as well as high vascularity and lymphatic drainage to the region, it is difficult for pathogens to infect the thyroid tissue. Despite all this, a persistent fistula from the piriform sinus may make the left lobe of the thyroid susceptible to infection and abscess formation. AIT is most often caused by a bacterial infection but can also be caused by a fungal or parasitic infection, most commonly in an immunocompromised host.

Dientamoebiasis is a medical condition caused by infection with "Dientamoeba fragilis", a single-cell parasite that infects the lower gastrointestinal tract of humans. It is an important cause of traveler's diarrhea, chronic abdominal pain, chronic fatigue, and failure to thrive in children.

In about 50-70% of patients with dyspepsia, no definite organic cause can be determined. In this case, dyspepsia is referred to as non-ulcer dyspepsia and its diagnosis is established by the presence of epigastralgia for at least 6 months, in the absence of any other cause explaining the symptoms.

Patients with atrophic gastritis are also at increased risk for the development of gastric adenocarcinoma. The optimal endoscopic surveillance strategy is not known but all nodules and polyps should be removed in these patients.

In the stomach there is a slight balance between acid and the wall lining which is protected by mucus. When this mucus lining is disrupted for whatever reason, signs and symptoms of acidity result. This may result in upper abdominal pain, indigestion, loss of appetite, nausea, vomiting and heartburn. When the condition is allowed to progress, the pain may become continuous; blood may start to leak and be seen in the stools. If the bleeding is rapid and of adequate volume it may even result in vomiting of bright red blood (hematemesis). When the acidity is uncontrolled, it can even cause severe blood loss (anemia) or lead to perforation (hole) in the stomach which is a surgical emergency. In many individuals, the progressive bleeding from an ulcer mixes with the feces and presents as black stools. Presence of blood in stools is often the first sign that there is a problem in the stomach.