Diarrhea can be defined as loose, watery stools, or bowel movements, for three or more times in a single day.

Diarrhea can be categorized into three types: Acute diarrhea that lasts for two weeks or less, persistent diarrhea that lasts for longer than two weeks and less than four weeks, and chronic diarrhea that persists from more than two weeks or is intermittent over months or years.

There are many causes that can lead to diarrhea, most important of which is contamination, poor hygiene and other factors that may lead to infection.

A large number of studies have identified possible risk factors for diarrhea that include early weaning, lack of piped water supply, poor water-storage practices, and unsatisfactory garbage disposal.

The pathogenesis of diarrhea manifests by one or more of five mechanisms. These mechanisms are osmotic, which results from malabsorption; secretory, when water secretion exceeds absorption; inflammatory and infectious, resulting from a virus or bacteria; diarrhea associated with deranged motility, which affects absorption; and diarrhea resulting from mucus, blood, or protein entering the bowel.

The signs and symptoms associated with diarrhea may include loose, watery stools, and an urgent need to have a bowel movement, in addition to abdominal cramps.

Health care providers may follow one or more diagnostic measure to find the cause of the illness; these may include testing stool or blood to search for bacteria, parasites, or other signs of a disease or infection involved. The doctor may recommend stopping eating certain foods to observe the progress of diarrhea, or may perform a colonoscopy to ascertain the cause of persistent diarrhea.

Most cases of diarrhea improve without special treatment within a couple of days, but in case the condition does not show any improvement, the healthcare provider may recommend other treatments depending on the cause, such antibiotics if diarrhea is the result of a bacterial infection.

Diarrhea can lead to a number of complications, such as dehydration, irritable bowel syndrome, and lactose intolerance.

Diarrhea can persist for several days, and can lead to a deficiency in water and salts essential for human survival; this may become life-threatening. From an epidemiological perspective, diarrhea-causing diseases are responsible for the death of around 760,000 children every year and are considered the second leading cause of death in children under 5 years old, according to the World Health Organization.

There is controversy regarding a unified definition for diarrhea, but mainly diarrhea can be defined as a loose, watery bowel movements that occur three or more times in one day. Frequent bowel movements of formed stools are not considered diarrhea. In addition, in case of breastfed babies, the passing of loose stools is not considered diarrhea.

Diarrhea can be categorized into three types:

Acute diarrhea: Acute diarrhea lasts for less than 2 weeks, and is self-limited, requiring no particular diagnostic procedures because its symptoms can be cured only by increasing fluid intake. Further investigation is only required when finding evidence of tissue invasions, such as blood or pus in the stool, leukocytosis, fever, and severity that generates considerable fluid and electrolyte loss. Acute diarrhea is usually the result of a bacterial, viral or parasitic infection.

Chronic diarrhea: Chronic diarrhea demands proper diagnostic procedures because it can last longer than 2 weeks, or can be intermittent over months or years.  This type is usually the result of an underlying condition, mainly inflammatory bowel diseases, such as ulcerative colitis.

Persistent diarrhea: This form of diarrhea lasts between two and four weeks.

There are many causes that can lead to diarrhea. The following are the most common:

• Infection: A broad range of bacterial, viral and parasitic organisms can result in diarrhea. The most common etiological agents of diarrhea in developing countries are Rotavirus and Escherichia coli.
• Malnutrition: Underlying malnutrition can make children more vulnerable to diarrhea.
• Contaminations: The water from sewage and latrines which may contain remnants of human or animal feces is an important cause of diarrhea.
• Poor personal hygiene: Poor personal hygiene can contribute to the spread of diarrhea-causing diseases from person-to-person. If food prepared or stored in unhygienic conditions, it is likely to become a major cause of diarrhea.
• Some treatments: Some medications, such as antibiotics, and abdominal surgeries can cause diarrhea as a side effect.
• Other conditions: Diarrhea may be caused by several conditions, such food allergies, food intolerances and conditions that affect the digestive tract, such as irritable bowel syndrome.

The following are risk factors that increase the possibility of getting diarrhea:

1. Infrequent hand washing with soap
2. Unsatisfactory garbage disposal, especially child feces
3. Lack of piped water supply
4. Poor water-storage and water boiling practices
5. Early weaning
6. Being male
7. Younger age
8. Low maternal education
9. Younger maternal age

Diarrhea can happen due to one of five basic mechanisms. It is common for more than one of these mechanisms to be involved in the pathogenesis of a single case:

• Osmotic Diarrhea: Osmotic diarrhea is generally caused by malabsorption. This can result from ingestion of a poorly absorbed substance, such Epsom salt, or can result from conditions that affect absorption, such as lactose intolerance which prevents dairy products containing lactose from being digested properly.
• Secretory Diarrhea: This form of diarrhea occurs when secretion of water into the intestines exceeds absorption. Diarrhea originating from cholera is a clear example of this type. A considerable number of other agents can induce secretory diarrhea, including some laxatives, and certain drugs, such antidepressants.
• Inflammatory and infectious diarrhea: Microbial or viral pathogens can cause damage to the inner lining of the intestine, which results in inefficient absorption of water leading to diarrhea.
• Diarrhea associated with disturbed motility: Disorders affecting intestinal motility could decrease absorption, resulting in diarrhea. Diarrhea, in this case, results from intestinal contents moving too fast through the intestine, which leads to inadequate absorption of water.
• Discharge of mucus, blood, and protein into the bowel: Inflammation and ulceration of the intestine may lead to the discharge of mucus, proteins, or blood into the bowel, which can lead to diarrhea.

Signs and symptoms associated with diarrhea may include:

• Loose, watery stools
• Urgent need to have a bowel movement
• Bloating and abdominal cramps
• Fever and nausea
• Abdominal pain

Diarrhea caused by some infections may trigger one or more of the following symptoms:

• Fever and chills
• Light-headedness and dizziness
• Vomiting
• Blood in the stools

The healthcare provider may conduct one or more of the following in order to find the cause of diarrhea:

• Physical examination: This may help to find additional clues to determine the cause of diarrhea. For instance, eye findings, such as protrusion of the eyeballs, may attribute diarrhea to inflammatory bowel disease and hyperthyroidism.
• Medication: The doctor may ask the patient about any medicines used because certain medications can cause diarrhea.
• Tests: The doctor may perform stool or blood tests to look for bacteria, parasites, or other signs of disease or infection that may be an underlying cause of diarrhea.
• Food restriction: The patient may be asked to stop eating certain foods to observe the progress of diarrhea as some foods can cause diarrhea in some people.
• Other diagnostic techniques: The healthcare provider may conduct sigmoidoscopy or colonoscopy to examine the lining of the colon, or take biopsies if no cause is clear for persistent diarrhea.

Most cases of diarrhea improve without special treatment within a couple of days. If the patient does not recover, the healthcare provider might recommend:

• Antibiotics: This kind of medications might help treat diarrhea caused by bacteria or parasites, but will not help if the diarrhea is caused by a virus.
• Fluid replacement: The healthcare provider is likely to recommend replacing fluids and salts by drinking water, juice or broth. If drinking liquids has bad consequences or causes more diarrhea, then, the healthcare provider might recommend intravenous fluid replacement.
• Adjusting medications used by the patient: If the healthcare provider confirms that a drug is the cause of diarrhea, he or she might lower the dose or recommend using another medication.

Diarrhea can lead to a number of complications, such as:

• Reactive complications by triggering other diseases, such as reactive arthritis.
• Dehydration and electrolyte imbalance.
• Spread of infection if safety measures are not taken seriously.
• Hemolytic uremic syndrome which is characterized by hemolytic anemia (anemia resulting from destruction of red blood cells), low platelet count, and acute kidney failure.
• Irritable bowel syndrome which is characterized by a group of symptoms consisting of abdominal pain and abnormalities in the pattern of bowel movements without any evidence of underlying damage.
• Serious consequences of reducing drug absorption, such as anti-epileptic drugs.
• Lactose intolerance.

Some of the key measures to prevent diarrhea include:

• Hand washing with soap.
• Access to safe drinking water.
• Use of improved sanitation.
• Exclusive breastfeeding for the first 6 months of life protects the baby.
• Health education about how infections spread.
• Rotavirus vaccination.
• Good personal and food hygiene.

Diarrhea can persist for several days, and it can cause a deficiency in water and salts that are essential for human survival. Dehydration and fluid loss are potentially fatal in patients diagnosed with severe diarrhea.

An improvement within 2-4 days could be noticed in many people with symptoms of acute diarrhea. Infections with Campylobacter spp. and Salmonella spp. usually last for 2-7 days. Rotavirus diarrhea usually lasts 3-8 days, and norovirus around 2 days.

According to the World Health Organization (WHO), diseases that cause diarrhea are responsible for killing around 760,000 children every year, making them the second leading cause of death in children under 5 years old. In addition, diarrhea is responsible for more than 80% of child deaths in Africa and South Asia.

The leading cause of acute diarrhea is rotavirus according to WHO, and rotavirus is responsible for about 40% of all hospital admissions due to diarrhea among children under five worldwide.