This condition can be a symptom of injury, disease or foodborne illness and is usually accompanied by abdominal pain, and often nausea and vomiting. There are other conditions which involve some but not all of the symptoms of diarrhea, and so the formal medical definition of diarrhea involves defecation of more than 200 grams per day (though formal weighing of stools to determine a diagnosis is never actually carried out).

It occurs when insufficient fluid is absorbed by the colon. As part of the digestion process, or due to fluid intake, food is mixed with large amounts of water. Thus, digested food is essentially liquid prior to reaching the colon. The colon absorbs water, leaving the remaining material as a semisolid stool. If the colon is damaged or inflamed, however, absorption is inhibited, and watery stools result.

Diarrhea is most commonly caused by myriad viral infections but is also often the result of bacterial toxins and sometimes even infection. In sanitary living conditions and with ample food and water available, an otherwise healthy patient typically recovers from the common viral infections in a few days and at most a week. However, for ill or malnourished individuals diarrhea can lead to severe dehydration and can become life-threatening without treatment.

It can also be a symptom of more serious diseases, such as dysentery, cholera, or botulism and can also be indicative of a chronic syndrome such as Crohn's disease. It is also an effect of severe radiation sickness.

It can also be caused by excessive alcohol consumption, especially in someone who doesn't eat enough food.

Symptomatic treatment for diarrhea involves the patient consuming adequate amounts of water to replace that lost, preferably mixed with electrolytes to provide essential salts and some amount of nutrients. For many people, further treatment and formal medical advice is unnecessary. The following types of diarrhea generally indicate medical supervision is desirable:

• Diarrhea in infants.
• Moderate or severe diarrhea in young children.
• Diarrhea associated with blood.
• Diarrhea that continues for more than 2 weeks.
• Diarrhea that is associated with more general illness such as non-cramping abdominal pain, fever, weight loss, etc.
• Diarrhea in travelers (more likely to have exotic infections such as parasites)
• Diarrhea in food handlers (potential to infect others)
• Diarrhea in institutions (Hospitals, child care, mental health institutes, geriatric and convalescent homes etc).

Since most people will ignore very minor diarrhea, a patient who actually presents to a doctor is likely to have diarrhea that is more severe than usual.

Acute diarrhea

This may defined as diarrhea that lasts less than 2 weeks, and is also called gastroenteritis.

This can nearly always be presumed to be infective although this is proven in a minority of cases.

It is often reasonable to reassure a patient, ensure adequate fluid intake and wait and see. In more severe cases or where it is important to find the cause of the illness stool cultures are instituted.

The most common organisms found are Campylobacter (an organism of animal or chicken origin), salmonella (also often of animal origin), Cryptosporidiosis (animal origin), Giardia Lamblia (lives in drinking water). Shigella (dysentery) is less common and usually human in origin. Cholera is rare in Western countries. It is more common in travelers and is usually related to contaminated water (its ultimate source is probably sea water). Escherichia coli is probably a very common cause of diarrhea, especially in travelers, but it can be difficult to detect using current technology. The types of E. coli vary from area to area and country to country.

Viruses, particularly rotavirus, are common in children. (Viral diarrhea is probably over-diagnosed by non-doctors). The Norwalk virus is rare.

Toxins and food poisoning can cause diarrhea. These include staphylococcal toxin (see Staphylococcus)(often milk products due to an infected wound in workers), and Bacillus cereus (eg rice in Chinese takeaways). Often "food poisoning" is really salmonella infection.

Diarrhea is a common side effect of drugs (especially antibiotics). Clostridium difficile infection is potentially serious and is often related to antibiotic use.

Parasites and worms sometime cause diarrhea but often present with weight loss, irritability, rashes or anal itching. The commonest is pinworm (mostly of nuisance value rather than a severe medical illness). Other worms such as hook worm, ascaris and tapeworm are more medically significant and may cause weight loss, anemia, general unwellness and allergic problems. Amoebic dysentery due to Entaeomeba histolytica is an important cause of bloody diarrhea in travelers and also sometimes in western countries which requires appropriate and complete medical treatment.

Chronic diarrhea

Infective diarrhea

It is not uncommon for diarrhea to persist. Diarrhea due to some organisms may persist for years without significant long term illness. More commonly a diarrhea will slowly ameliorate but the patient becomes a carrier (harbors the infection without illness). This is often an indication for treatment, especially in food workers or institution workers.

Parasites (worms and amoeba) should always be treated. Salmonella is the most common persistent bacterial organism in humans.

Non infective diarrhea

These tend to be more severe medical illnesses. Malabsorption: This is due to the inability to absorb food, mostly in the small bowel but also due to the pancreas.

Causes include celiac disease (intolerance to gluten, a wheat product), lactose intolerance (Intolerance to milk sugar, common in non-Europeans), fructose malabsorption, Pernicious anemia (impaired bowel function due to the inability to absorb vitamin B12), loss of pancreatic secretions (may be due to cystic fibrosis or pancreatitis), short bowel syndrome (surgically removed bowel), radiation fibrosis (usually following cancer treatment), other drugs such as chemotherapy, and of course, diarrhea-predominant irritable bowel syndrome.

Chronic inflammatory diseases

There are of unknown origin but are likely to be abnormal immune responses to infection. There is some overlap but the two types are ulcerative colitis and Crohn's disease.

Ulcerative colitis is marked by chronic bloody diarrhea and inflammation mostly affects the distal colon near the rectum.

Crohn's disease typically affects fairly well demarcated segments of bowel in the colon and often affects the end of the small bowel.

Other important causes

• Ischaemic bowel disease. This usually affects older people and can be due to blocked arteries.
• Bowel cancer: Some (but NOT all) bowel cancers may have associated diarrhea. (Cancer of the large colon is most common)
• Hormone-secreting tumors: Some hormones can cause diarrhea if excreted to excess (usually from a tumor).

Treatment of diarrhea

1. Do nothing except ensure adequate fluids. This is the most appropriate treatment in most cases of minor diarrhea.
2. Try eating more but smaller portions. Eat regularly. Don't eat or drink too quickly.
3. Anti-diarrhea drugs: Use cautiously as they are said to prolong the illness and may increase the risk of a carrier state. They are useful in some cases, however, when it is important that you don't have diarrhea (e.g. when traveling on a bus).
4. Antibiotics: Antibiotics may be required if they can be effective and the patient is medically ill. They are sometimes also indicated for workers with carrier states in order to clear up an infection so that the person can resume work. Parasites require appropriate antibiotics.
5. Intravenous fluids or a "drip": Sometimes, especially in children, dehydration can be life-theatening and intravenous fluid may be required.
6. Oral rehydration therapy: Taking a sugar/salt solution, which can be absorbed by the body.
7. Dietary manipulation: especially avoid wheat products with celiac disease.
8. Hygiene and sometime isolation: Hygiene is important in limiting spread of the disease.
9. It is claimed that some fruit, such as bananas, mangoes, papaya and pineapple may have positive effects on this condition. Bananas have the merits of being easily obtainable, and they are unlikely to have any other significant unwanted side effects. Bananas are thought to be "binding," as is mucilage, which you can obtain in capsule form. Mucilage can also be used as cereal for babies, as it is easily digested. The high acid content of pineapple may make the tasty tropical treat a bad choice for people suffering from chronic, explosive diarrhoea.