Colitis-X is a term used for colitis cases in which no definitive diagnosis can be made and the horse dies. Clinical signs include sudden, watery diarrhea that is usually accompanied by symptoms of hypovolemic shock and usually leads to death in 3 to 48 hours, usually in less than 24 hours. Other clinical signs include tachycardia, tachypnea, and a weak pulse. Marked depression is present. An explosive diarrhea develops, resulting in extreme dehydration. Hypovolemic and endotoxic shock are manifest by increased capillary refill time, congested or cyanotic (purplish) mucous membranes, and cold extremities. While there may initially be a fever, temperature usually returns to normal.

Clinical signs are similar to those of other diarrheal diseases, including toxemia caused by "Clostridium", Potomac horse fever, experimental endotoxic shock, and anaphylaxis.

Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel. Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B can develop a megaloblastic anemia with large red blood cells.

In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.

The onset of TD usually occurs within the first week of travel, but may occur at any time while traveling, and even after returning home, depending on the incubation period of the infectious agent. Bacterial TD typically begins abruptly, but "Cryptosporidium" may incubate for seven days, and "Giardia" for 14 days or more, before symptoms develop. Typically, a traveler experiences four to five loose or watery bowel movements each day. Other commonly associated symptoms are abdominal cramping, bloating, fever, and malaise. Appetite may decrease significantly. Though unpleasant, most cases of TD are mild, and resolve in a few days without medical intervention.

Blood or mucus in the diarrhea, significant abdominal pain, or high fever suggests a more serious cause, such as cholera, characterized by a rapid onset of weakness and torrents of watery diarrhea with flecks of mucus (described as "rice water" stools). Medical care should be sought in such cases; dehydration is a serious consequence of cholera, and may trigger serious sequelae—including, in rare instances, death—as rapidly as 24 hours after onset if not addressed promptly.

Small intestine bacterial overgrowth (SIBO), also termed bacterial overgrowths, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small bowel usually has fewer than 10,000 organisms per millilitre. Patients with bacterial overgrowth typically develop symptoms including nausea, bloating, vomiting, diarrhea, malnutrition, weight loss and malabsorption, which is caused by a number of mechanisms.

The diagnosis of bacterial overgrowth is made by a number of techniques, with the gold standard being an aspirate from the jejunum that grows in excess of 10 bacteria per millilitre. Risk factors for the development of bacterial overgrowth include dysmotility; anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve; gastroenteritis-induced alterations to the small intestine; and the use of certain medications, including proton pump inhibitors.

Small bowel bacterial overgrowth syndrome is treated with an elemental diet or antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics, sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.

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".

Colitis X, equine colitis X or peracute toxemic colitis is a catchall term for various fatal forms of acute or peracute colitis found in horses, but particularly a fulminant colitis where clinical signs include sudden onset of severe diarrhea, abdominal pain, shock, and dehydration. Death is common, with 90% to 100% mortality, usually in less than 24 hours. The causative factor may be "Clostridium difficile", but it also may be caused by other intestinal pathogens. Horses under stress appear to be more susceptible to developing colitis X, and like the condition pseudomembranous colitis in humans, there also is an association with prior antibiotic use. Immediate and aggressive treatment can sometimes save the horse, but even in such cases, 75% mortality is considered a best-case scenario.

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.

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.

Traveler's diarrhea (TD) is a stomach and intestinal infection. TD is defined as the passage of unformed stool (one or more by some definitions, three or more by others) while traveling. It may be accompanied by abdominal cramps, nausea, fever, and bloating. Occasionally bloody diarrhea may occur. Most travelers recover within four days with little or no treatment. About 10% of people may have symptoms for a week.

Bacteria are responsible for more than half of cases. The bacteria enterotoxigenic "Escherichia coli" (ETEC) are typically the most common except in Southeast Asia, where "Campylobacter" is more prominent. About 10% to 20% of cases are due to norovirus. Protozoa such as "Giardia" may cause longer term disease. The risk is greatest in the first two weeks of travel and among young adults. People affected are more often from the developed world.

Recommendations for prevention include eating only properly cleaned and cooked food, drinking bottled water, and frequent hand washing. The oral cholera vaccine, while effective for cholera, is of questionable use for traveler's diarrhea. Preventative antibiotics are generally discouraged. Primary treatment includes drinking lots of fluids and replacing lost salts (oral rehydration therapy). Antibiotics are recommended for significant or persistent symptoms, and can be taken with loperamide to decrease diarrhea. Hospitalization is required in less than 3% of cases.

Estimates of the percentage of people affected range from 20 to 50% among travelers to the developing world. TD is particularly common among people travelling to Asia (except Japan), the Middle East, Africa, Mexico, and Central and South America. The risk is moderate in Southern Europe, Russia, and China. TD has been linked to later irritable bowel syndrome and Guillain–Barré syndrome. It has colloquially been known by a number of names, including "Montezuma's revenge" and "Delhi belly".

Enteritis is inflammation of the small intestine. It is most commonly caused by food or drink contaminated with pathogenic microbes. but may have other causes such as NSAIDs, cocaine, radiation therapy as well as autoimmune conditions like Crohn's disease and coeliac disease. Symptoms include abdominal pain, cramping, diarrhoea, dehydration, and fever. Related diseases include inflammation of the stomach (gastritis) and large intestine (colitis).

Duodenitis, jejunitis and ileitis are subtypes of enteritis which are only localised to a specific part of the small intestine. Inflammation of both the stomach and small intestine is referred to as gastroenteritis. Inflammation of related organs of the gastrointestinal system are:

- gastritis

- gastroenteritis

- colitis

- enterocolitis

The average incubation periods for giardiasis and cryptosporidiosis are each 7 days. Certain other bacterial and viral agents have shorter incubation periods, although hepatitis may take weeks to manifest itself. The onset usually occurs within the first week of return from the field, but may also occur at any time while hiking.

Most cases begin abruptly and usually result in increased frequency, volume, and weight of stool. Typically, a hiker experiences at least four to five loose or watery bowel movements each day. Other commonly associated symptoms are nausea, vomiting, abdominal cramping, bloating, low fever, urgency, and malaise, and usually the appetite is affected. The condition is much more serious if there is blood or mucus in stools, abdominal pain, or high fever. Dehydration is a possibility. Life-threatening illness resulting from WAD is extremely rare but can occur in people with weakened immune systems.

Some people may be carriers and not exhibit symptoms.

Symptoms vary from none to severe diarrhea with poor absorption of nutrients. It can result in weakness, loss of appetite, stomach cramps, vomiting, bloating, excessive gas, and burping. Symptoms typically develop 9–15 days after exposure, but may occur as early as one day.

Symptoms are caused by "Giardia" organisms infecting the cells of the duodenum and jejunum of the small intestine and blocking nutrient absorption. Most people are asymptomatic; only about a third of those infected exhibit symptoms. If the infection is not treated, these symptoms may last for six weeks or more.

Symptomatic infections are well recognized as causing lactose intolerance, which, while usually temporary, may become permanent. Although hydrogen breath tests indicate poorer rates of carbohydrate absorption in those asymptomatically infected, such tests are not diagnostic of infection. It has been suggested that these observations are explained by symptomatic giardia infection allowing for the overgrowth of other bacteria.

Some studies have shown giardiasis should be considered as a cause of vitamin B deficiency as a result of the problems caused within the intestinal absorption system.

Gastroenteritis typically involves both diarrhea and vomiting, or less commonly, presents with only one or the other. Abdominal cramping may also be present. Signs and symptoms usually begin 12–72 hours after contracting the infectious agent. If due to a viral agent, the condition usually resolves within one week. Some viral causes may also be associated with fever, fatigue, headache, and muscle pain. If the stool is bloody, the cause is less likely to be viral and more likely to be bacterial. Some bacterial infections may be associated with severe abdominal pain and may persist for several weeks.

Children infected with rotavirus usually make a full recovery within three to eight days. However, in poor countries treatment for severe infections is often out of reach and persistent diarrhea is common. Dehydration is a common complication of diarrhea, and a child with a significant degree of dehydration may have a prolonged capillary refill, poor skin turgor, and abnormal breathing. Repeat infections are typically seen in areas with poor sanitation, and malnutrition, stunted growth, and long-term cognitive delays can result.

Reactive arthritis occurs in 1% of people following infections with "Campylobacter" species, and Guillain–Barré syndrome occurs in 0.1%. Hemolytic uremic syndrome (HUS) may occur due to infection with Shiga toxin-producing "Escherichia coli" or "Shigella" species, causing low platelet counts, poor kidney function, and low red blood cell count (due to their breakdown). Children are more predisposed to getting HUS than adults. Some viral infections may produce benign infantile seizures.

Diarrhea acquired in the wilderness or other remote areas is typically a form of infectious diarrhea, itself classified as a type of secretory diarrhea. These are all considered forms of gastroenteritis. The term may be applied in various remote areas of non-tropical developed countries (U.S., Canada, western Europe, etc.), but is less applicable in developing countries, and in the tropics, because of the different pathogens that are most likely to cause infection.

The signs and symptoms of colitis are quite variable and dependent on the cause of the given colitis and factors that modify its course and severity.

Symptoms of colitis may include: mild to severe abdominal pain and tenderness (depending on the stage of the disease), recurring bloody diarrhea with/without pus in the stools, fecal incontinence, flatulence, fatigue, loss of appetite and unexplained weight loss.

More severe symptoms may include: shortness of breath, a fast or irregular heartbeat and fever.

Other less or rare non-specific symptoms that may accompany colitis include: arthritis, mouth ulcers, painful, red and swollen skin and irritated, red eyes.

Signs seen on colonoscopy include: colonic mucosal erythema (redness of the inner surface of the colon), ulcers, and bleeding.

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.

Giardiasis, popularly known as beaver fever, is a parasitic disease caused by "Giardia lamblia". About 10% of those infected have no symptoms. When symptoms occur they may include diarrhea, abdominal pain, and weight loss. Vomiting, blood in the stool, and fever are less common. Symptoms usually begin 1 to 3 weeks after exposure and without treatment may last up to six weeks.

Giardia usually spreads when "Giardia lamblia" cysts within feces contaminate food or water which is then eaten or drunk. It may also spread between people and from other animals. Risk factors include travel in the developing world, changing diapers, eating food without cooking it, and owning a dog. Cysts may survive for nearly three months in cold water. Diagnosis is via stool tests.

Prevention is typically by improved hygiene. Those without symptoms do not usually need treatment. When symptoms are present treatment is typically with either tinidazole or metronidazole. People may become temporarily lactose intolerant after an infection and therefore it is often recommended milk be avoided for a few weeks. Resistance to treatment may occur.

Giardia is one of the most common parasitic human diseases globally. In 2013, there were about 280 million people worldwide with symptomatic giardiasis. Rates are as high as 7% in the developed world and 30% in the developing world. The World Health Organization classified it as a neglected disease.

The condition is usually caused by Gram-positive enteric commensal bacteria of the gut (gut flora). "Clostridium difficile" is a species of Gram-positive bacteria that commonly causes severe diarrhea and other intestinal diseases when competing bacteria are wiped out by antibiotics, causing pseudomembranous colitis, whereas Clostridium septicum is responsible for most cases of neutropenic enterocolitis.

Typhlitis most commonly occurs in immunocompromised patients, such as those undergoing chemotherapy, patients with AIDS, kidney transplant patients, or the elderly.

CNE is a necrotizing inflammation of the small bowel (especially the jejunum but also the ileum). Clinical results may vary from mild diarrhea to a life-threatening sequence of severe abdominal pain, vomiting, bloody stool, ulceration of the small intestine with leakage (perforation) into the peritoneal cavity and possible death within a single day due to peritonitis. Many patients exhibit meteorism. Treatment involves suppressing the toxin-producing organisms with antibiotics such as penicillin G or metronidazole. About half of seriously ill patients require surgery for perforation, persistent intestinal obstruction, or failure to respond to the antibiotics. An investigational toxoid vaccine has been used successfully in some developing countries but is not available outside of research.

There are many types of colitis. They are usually classified by the cause.

Types of colitis include:

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

All the factors collectively causing CNE are generally only present in the hinterlands of New Guinea and parts of Africa, Latin America, and Asia. These factors include protein deprivation (causing inadequate synthesis of trypsin protease (an enzyme), to which the toxin is very sensitive), poor food hygiene, episodic meat feasting, staple diets containing trypsin inhibitors (sweet potatoes), and infection by "Ascaris" parasites which secrete a trypsin inhibitor. In New Guinea (origin of the term "pigbel"), the disease is usually spread through contaminated meat (especially pork) and perhaps by peanuts. (CNE was also diagnosed in post World War II Germany, where it was known as "Darmbrand" or "fire bowels").

Gastroenteritis, also known as infectious diarrhea, is inflammation of the gastrointestinal tract that involves the stomach and small intestine. Symptoms may include diarrhea, vomiting, and abdominal pain. Fever, lack of energy, and dehydration may also occur. This typically lasts less than two weeks. It is not related to influenza though it has been called the "stomach flu".

Gastroenteritis can be due to infections by viruses, bacteria, parasites, and fungus. The most common cause is viruses. In children rotavirus is the most common cause of severe disease. In adults, norovirus and "Campylobacter" are common. Transmission may occur due to eating improperly prepared foods, drinking contaminated water, or through close contact with an individual who is infected. Testing to confirm the diagnosis is typically not needed.

Prevention includes hand washing with soap, drinking clean water, proper disposal of human waste, and breastfeeding babies instead of using formula. The rotavirus vaccine is recommended in children. Treatment involves getting enough fluids. For mild or moderate cases, this can typically be achieved by drinking oral rehydration solution (a combination of water, salts, and sugar). In those who are breast fed, continued breastfeeding is recommended. For more severe cases, intravenous fluids may be needed. Fluids may also be given by a nasogastric tube. Zinc supplementation is recommended in children. Antibiotics are generally not needed.

In 2015 two billion cases of gastroenteritis resulted in 1.3 million deaths globally. Children and those in the developing world are affected the most. In 2011, about 1.7 billion cases resulting in about 700,000 deaths of children under the age of five. In the developing world children less than two years of age frequently get six or more infections a year. It is less common in adults, partly due to the development of immunity.

Antibiotic-associated diarrhea (AAD) results from an imbalance in the colonic microbiota caused by antibiotic therapy. Microbiota alteration changes carbohydrate metabolism with decreased short-chain fatty acid absorption and an osmotic diarrhea as a result. Another consequence of antibiotic therapy leading to diarrhea is overgrowth of potentially pathogenic organisms such as "Clostridium difficile". It is defined as frequent loose and watery stools with no other complications.

Meta-analyses have concluded that probiotics may protect against antibiotic-associated diarrhea in both children and adults. Evidence is insufficient, however, regarding an effect on rates of "Clostridium difficile" colitis.

However, citing conflicting data in the studies, other sources claim that the use of probiotics has failed thus far to meet the standard of medical care required for evidence-based medicine. Demonstration of the efficacy of probiotics is needed by randomized, double blind, placebo-controlled trials.

Efficacy of probiotic AAD prevention is dependent on the probiotic strain(s) used and on the dosage. Up to a 50% reduction of AAD occurrence has been found. No side-effects have been reported in any of these studies. Caution should, however, be exercised when administering probiotic supplements to immunocompromised individuals or patients who have a compromised intestinal barrier because of the risk of an infection caused by the probiotic supplements.

"Clostridium difficile", also known more commonly as "C. diff", is known to account for 10 to 20 percent of antibiotic-associated diarrhea cases. The reasoning for this, is that the antibiotics administered for the treatment of certain diseases processes such as inflammatory colitis also inadvertently kills a large portion of the gut flora, the normal flora that is usually present within the bowel. With this lower amount of "healthy" bacteria present, the overgrowth of "C. diff" is then responsible "for elaborating the enterotoxin".

In adults the most common source of aspiration pneumonia is aspiration of oropharyngeal secretions or gastric contents. In children the most common cause is aspiration of infected amniotic fluid, or vaginal secretions. Severe periodontal or gingival diseases are important risk factors for establishment of an anaerobic pleuropulmonary infection. Progression of the infection from pneumonitis into necrotizing pneumonia and pulmonary abscess can occur, with or without the development of empyema., The infection is often polymicrobial in nature and isolates of community-acquired infection (in 60–80% of cases) are aerobic and anaerobic belonging to the individual's oropharyngeal flora. The anaerobic bacteria commonly recovered are "Prevotella, Porphyromonas, Fusobacterium" and "Peptostreptococcus" spp., and the aerobic bacteria are beta-hemolytic and microaerophilic streptococci. Anaerobic bacteria can also be isolated in about 35% of individuals who suffer from nosocomial-acquired aspiration pneumonia and pneumonia associated with tracheostomy with and without mechanical ventilation, where they are often isolated along with Enterobacteriaceae, "Pseudomonas" spp. and "Staphylococcus aureus". It is important that specimens are obtained in a method that avoids their contamination by the oral micro flora.