One of the most common causes of infectious diarrhea, is a lack of clean water. Often, improper fecal disposal leads to contamination of groundwater. This can lead to widespread infection among a population, especially in the absence of water filtration or purification. Human feces contains a variety of potentially harmful human pathogens.

Open defecation is a leading cause of infectious diarrhea leading to death.

Poverty is a good indicator of the rate of infectious diarrhea in a population. This association does not stem from poverty itself, but rather from the conditions under which impoverished people live. The absence of certain resources compromises the ability of the poor to defend themselves against infectious diarrhea. "Poverty is associated with poor housing, crowding, dirt floors, lack of access to clean water or to sanitary disposal of fecal waste (sanitation), cohabitation with domestic animals that may carry human pathogens, and a lack of refrigerated storage for food, all of which increase the frequency of diarrhea... Poverty also restricts the ability to provide age-appropriate, nutritionally balanced diets or to modify diets when diarrhea develops so as to mitigate and repair nutrient losses. The impact is exacerbated by the lack of adequate, available, and affordable medical care."

Travelers often get diarrhea from eating and drinking foods and beverages that have no adverse effects on local residents. This is due to immunity that develops with constant, repeated exposure to pathogenic organisms. The extent and duration of exposure necessary to acquire immunity has not been determined; it may vary with each individual organism. A study among expatriates in Nepal suggests that immunity may take up to seven years to develop—presumably in adults who avoid deliberate pathogen exposure.

Conversely, immunity acquired by American students while living in Mexico disappeared, in one study, as quickly as eight weeks after cessation of exposure.

The primary source of infection is ingestion of fecally contaminated food or water. Attack rates are similar for men and women.

The most important determinant of risk is the traveler's destination. High-risk destinations include developing countries in Latin America, Africa, the Middle East, and Asia. Among backpackers, additional risk factors include drinking untreated surface water and failure to maintain personal hygiene practices and clean cookware. Campsites often have very primitive (if any) sanitation facilities, making them potentially as dangerous as any developing country.

Although traveler's diarrhea usually resolves within three to five days (mean duration: 3.6 days), in about 20% of cases, the illness is severe enough to require bedrest, and in 10%, the illness duration exceeds one week. For those prone to serious infections, such as bacillary dysentery, amoebic dysentery, and cholera, TD can occasionally be life-threatening. Others at higher-than-average risk include young adults, immunosuppressed persons, persons with inflammatory bowel disease or diabetes, and those taking H2 blockers or antacids.

Transmission may occur via consumption of contaminated water, or when people share personal objects. In places with wet and dry seasons, water quality typically worsens during the wet season, and this correlates with the time of outbreaks. In areas of the world with four seasons, infections are more common in the winter. Bottle-feeding of babies with improperly sanitized bottles is a significant cause on a global scale. Transmission rates are also related to poor hygiene, especially among children, in crowded households, and in those with pre-existing poor nutritional status. After developing tolerance, adults may carry certain organisms without exhibiting signs or symptoms, and thus act as natural reservoirs of contagion. While some agents (such as "Shigella") only occur in primates, others may occur in a wide variety of animals (such as "Giardia").

A number of protozoa can cause gastroenteritis – most commonly "Giardia lamblia" – but "Entamoeba histolytica", "Cryptosporidium" spp., and other species have also been implicated. As a group, these agents comprise about 10% of cases in children. "Giardia" occurs more commonly in the developing world, but this etiologic agent causes this type of illness to some degree nearly everywhere. It occurs more commonly in persons who have traveled to areas with high prevalence, children who attend day care, men who have sex with men, and following disasters.

One study suggests that on very long trips in the wilderness, taking multivitamins may reduce the incidence of diarrhea.

Depending on the cause of the inflammation, symptoms may last from one day to more than a week.

Gastroenteritis caused by viruses may last one to two days. Most people recover easily from a short episode of vomiting and diarrhea by drinking clear fluids to replace the fluid that was lost and then gradually progressing to a normal diet. But for others, especially infants and the elderly, the loss of bodily fluid with gastroenteritis can cause dehydration, which can be a life-threatening illness unless it is treated and fluids in the body are replaced.

With most infections, the key is to block the spread of the organism.

- Wash hands frequently

- Eat properly prepared and stored food.

- Bleach soiled laundry

- Vaccinations for "Vibrio cholerae" and rotavirus have been developed. Rotavirus vaccination is recommended for infants in the U.S. Vaccines for "V. cholerae" may be administered to individuals traveling to at-risk areas

Insufficient data exists, but "Shigella" is estimated to have caused the death of 34,000 children under the age of five in 2013, and 40,000 deaths in people over five years of age. "Amebiasis" infects over 50 million people each year, of whom 50,000 die.

Although there is currently no vaccine which protects against "Shigella" infection, several are in development. Vaccination may eventually become a critical part of the strategy to reduce the incidence and severity of diarrhea, particularly among children in low-resource settings. For example, "Shigella" is a longstanding World Health Organization (WHO) target for vaccine development, and sharp declines in age-specific diarrhea/dysentery attack rates for this pathogen indicate that natural immunity does develop following exposure; thus, vaccination to prevent this disease should be feasible. The development of vaccines against these types of infection has been hampered by technical constraints, insufficient support for coordination, and a lack of market forces for research and development. Most vaccine development efforts are taking place in the public sector or as research programs within biotechnology companies.

The risk of acquiring infectious diarrhea in the wilderness arises from inadvertent ingestion of pathogens. Various studies have sought to estimate diarrhea attack rates among wilderness travelers, and results have ranged widely. The variation of diarrhea rate between studies may depend on the time of year, the location of the study, the length of time the hikers were in the wilderness,

the prevention methods used, and the study methodology.

The National Outdoor Leadership School (NOLS), which emphasizes strict hand-washing techniques, water disinfection and washing of common cooking utensils in their programs, reports that gastrointestinal illnesses occurred at a rate of only 0.26 per 1000 program days. In contrast, a survey of long-distance Appalachian Trail hikers found more than half the respondents reported at least one episode of diarrhea that lasted an average of two days. (Infectious diarrhea may last longer than an average of two days; certain forms of non-infectious diarrhea, caused by diet change etc., can be of very brief duration). Analysis of this survey found occurrence of diarrhea was positively associated with the duration of exposure in the wilderness. During any given four-week period, as many as 7.2% of Americans may experience some form of infectious or non-infectious diarrhea. A number of behaviors each individually reduced the incidence of diarrhea: treating water; routinely washing hands with soap and water after defecation and urination; cleaning cooking utensils with soap and warm water; and taking multi-vitamins.

A variety of pathogens can cause infectious diarrhea, and most cases among backpackers appear to be caused by bacteria from feces. A study at Grand Teton National Park found 69% of diarrhea affected visitors had no identifiable cause, that 23% had diarrhea due to "Campylobacter" and 8% of patients with diarrhea had giardiasis. Campylobacter enteritis occurred most frequently in young adults who had hiked in wilderness areas and drunk untreated surface water in the week prior. Another study tested 35 individuals before and after a trip to the Desolation Wilderness of California. Giardia cysts were found in fecal samples from two people after the trip, but they were asymptomatic. A third person was empirically treated for symptoms of giardiasis.

Fecal-oral transmission may be the most common vector for wilderness acquired diarrhea. There are differing opinions regarding the importance of routine disinfection of water during relatively brief backcountry visits.

"Salmonella" bacteria can survive for some time without a host; thus, they are frequently found in polluted water, with contamination from the excrement of carrier animals being particularly important.

The European Food Safety Authority highly recommends that when handling raw turkey meat, consumers and people involved in the food supply chain should pay attention to personal and food hygiene.

An estimated 142,000 Americans are infected each year with "Salmonella" Enteritidis from chicken eggs, and about 30 die. The shell of the egg may be contaminated with "Salmonella" by feces or environment, or its interior (yolk) may be contaminated by penetration of the bacteria through the porous shell or from a hen whose infected ovaries contaminate the egg during egg formation.

Nevertheless, such interior egg yolk contamination is theoretically unlikely. Even under natural conditions, the rate of infection was very small (0.6% in a study of naturally contaminated eggs and 3.0% among artificially and heavily infected hens).

The FDA has published guidelines to help reduce the chance of food-borne salmonellosis. Food must be cooked to 68–72 °C (145–160 °F), and liquids such as soups or gravies must be boiled. Freezing kills some "Salmonella", but it is not sufficient to reliably reduce them below infectious levels. While "Salmonella" is usually heat-sensitive, it does acquire heat resistance in high-fat environments such as peanut butter.

In the majority of cases, amoebas remain in the gastrointestinal tract of the hosts. Severe ulceration of the gastrointestinal mucosal surfaces occurs in less than 16% of cases. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Only rarely are masses formed (amoebomas) that lead to intestinal obstruction.(Mistaken for Ca caecum and appendicular mass) Other local complications include bloody diarrhea, pericolic and pericaecal abscess.

Complications of hepatic amoebiasis includes subdiaphragmatic abscess, perforation of diaphragm to pericardium and pleural cavity, perforation to abdominal cavital "(amoebic peritonitis)" and perforation of skin "(amoebiasis cutis)".

Pulmonary amoebiasis can occur from hepatic lesion by haemotagenous spread and also by perforation of pleural cavity and lung. It can cause lung abscess, pulmono pleural fistula, empyema lung and broncho pleural fistula. It can also reach the brain through blood vessels and cause amoebic brain abscess and amoebic meningoencephalitis. Cutaneous amoebiasis can also occur in skin around sites of colostomy wound, perianal region, region overlying visceral lesion and at the site of drainage of liver abscess.

Urogenital tract amoebiasis derived from intestinal lesion can cause amoebic vulvovaginitis "(May's disease)", rectovesicle fistula and rectovaginal fistula.

"Entamoeba histolytica" infection is associated with malnutrition and stunting of growth.

Amoebiasis is usually transmitted by the fecal-oral route, but it can also be transmitted indirectly through contact with dirty hands or objects as well as by anal-oral contact. Infection is spread through ingestion of the cyst form of the parasite, a semi-dormant and hardy structure found in feces. Any non-encysted amoebae, or "trophozoites", die quickly after leaving the body but may also be present in stool: these are rarely the source of new infections. Since amoebiasis is transmitted through contaminated food and water, it is often endemic in regions of the world with limited modern sanitation systems, including México, Central America, western South America, South Asia, and western and southern Africa.

Amoebic dysentery is often confused with "traveler's diarrhea" because of its prevalence in developing nations. In fact, most traveler's diarrhea is bacterial or viral in origin.

Cholera has been found in two animal populations: shellfish and plankton.

Transmission is usually through the fecal-oral route of contaminated food or water caused by poor sanitation. Most cholera cases in developed countries are a result of transmission by food, while in the developing world it is more often water. Food transmission can occur when people harvest seafood such as oysters in waters infected with sewage, as "Vibrio cholerae" accumulates in planktonic crustaceans and the oysters eat the zooplankton.

People infected with cholera often have diarrhea, and disease transmission may occur if this highly liquid stool, colloquially referred to as "rice-water", contaminates water used by others. A single diarrheal event can cause a one-million fold increase in numbers of "V. cholerae" in the environment. The source of the contamination is typically other cholera sufferers when their untreated diarrheal discharge is allowed to get into waterways, groundwater or drinking water supplies. Drinking any contaminated water and eating any foods washed in the water, as well as shellfish living in the affected waterway, can cause a person to contract an infection. Cholera is rarely spread directly from person to person.

Both toxic and non-toxic strains exist. Non-toxic strains can acquire toxicity through a temperate bacteriophage.

If people with cholera are treated quickly and properly, the mortality rate is less than 1%; however, with untreated cholera, the mortality rate rises to 50–60%.

For certain genetic strains of cholera, such as the one present during the 2010 epidemic in Haiti and the 2004 outbreak in India, death can occur within two hours of becoming ill.

Some studies reported up to 80% of patients with irritable bowel syndrome (IBS) have SIBO (using the hydrogen breath test). Subsequent studies demonstrated statistically significant reduction in IBS symptoms following therapy for SIBO.

There is a lack of consensus however, regarding the suggested link between IBS and SIBO. Other authors concluded that the abnormal breath results so common in IBS patients do not suggest SIBO, and state that "abnormal fermentation timing and dynamics of the breath test findings support a role for abnormal intestinal bacterial distribution in IBS." There is general consensus that breath tests are abnormal in IBS; however, the disagreement lies in whether this is representative of SIBO. More research is needed to clarifiy this possible link.

To date, the precise causative factor has not been verified, and the disease has been attributed by various sources to viruses, parasites, bacteria, use of antibiotics and sulfonamides, and heavy metal poisoning. Other possible causes include peracute salmonellosis, clostridial enterocolitis, and endotoxemia. "Clostridium difficile" toxins isolated in the horse have a genotype like the current human "epidemic strain", which is associated with human "C. difficile"-associated disease of greater than historical severity. "C. difficile" can cause pseudomembranous colitis in humans, and in hospitalized patients who develop it, fulminant "C. difficile" colitis is a significant and increasing cause of death.

Horses under stress appear to be more susceptible to developing colitis X. Disease onset is often closely associated with surgery or transport. Excess protein and lack of cellulose content in the diet (a diet heavy on grain and lacking adequate hay or similar roughage) is thought to be the trigger for the multiplication of clostridial organisms. A similar condition may be seen after administration of tetracycline or lincomycin to horses. These factors may be one reason the condition often develops in race horses, having been responsible for the deaths of the Thoroughbred filly Landaluce,

the Quarter Horse stallion Lightning Bar,

and is one theory for the sudden death of Kentucky Derby winner Swale.

The link to stress suggests the condition may be brought on by changes in the microflora of the cecum and colon that lower the number of anaerobic bacteria, increase the number of Gram-negative enteric bacteria, and decrease anaerobic fermentation of soluble carbohydrates, resulting in damage to the cecal and colonic mucosa and allowing increased absorption of endotoxins from the lumen of the gut.

The causative agent may be "Clostridium perfringens", type A, but the bacteria are recoverable only in the preliminary stages of the disease.

The suspect toxin could also be a form of "Clostridium difficile". In a 2009 study at the University of Arizona, "C. difficile" toxins A and B were detected, large numbers of "C. difficile" were isolated, and genetic characterization revealed them to be North American pulsed-field gel electrophoresis type 1, polymerase chain reaction ribotype 027, and toxinotype III. Genes for the binary toxin were present, and toxin negative-regulator tcdC contained an 18-bp deletion. The individual animal studied in this case was diagnosed as having peracute typhlocolitis, with lesions and history typical of those attributed to colitis X.

Use of antibiotics may also be associated with some forms of colitis-X. In humans, "C. difficile" is the most serious cause of antibiotic-associated diarrhea, often a result of eradication of the normal gut flora by antibiotics. In one equine study, colitis was induced after pretreatment with clindamycin and lincomycin, followed by intestinal content from horses which had died from naturally occurring idiopathic colitis. (A classic adverse effect of clindamycin in humans is "C. difficile"-associated diarrhea.) In the experiment, the treated horses died. After necropsy, "Clostridium cadaveris" was present, and is proposed as another possible causative agent in some cases of fatal colitis.

Smoking has been linked to a variety of disorders of the stomach. Tobacco is known to stimulate acid production and impairs production of the protective mucus. This leads to development of ulcers in the majority of smokers.

Chronic stomach problems have also been linked to excess intake of alcohol. It has been shown that alcohol intake can cause stomach ulcer, gastritis and even stomach cancer. Thus, avoidance of smoking and excess alcohol consumption can help prevent the majority of chronic stomach disorders.

One of the most causes of chronic stomach problems is use of medications. Use of aspirin and other non-steroidal anti-inflammatory drugs to treat various pain disorders can damage lining of the stomach and cause ulcers. Other medications like narcotics can interfere with stomach emptying and cause bloating, nausea, or vomiting.

The majority of chronic stomach problems are treated medically. However, there is evidence that a change in life style may help. Even though there is no specific food responsible for causing chronic stomach problems, experts recommend eating a healthy diet which consists of fruits and vegetables. Lean meat should be limited. Moreover, people should keep a diary of foods that cause problems and avoid them.

To date, no licensed vaccines specifically target ETEC, though several are in various stages of development. Studies indicate that protective immunity to ETEC develops after natural or experimental infection, suggesting that vaccine-induced ETEC immunity should be feasible and could be an effective preventive strategy. Prevention through vaccination is a critical part of the strategy to reduce the incidence and severity of diarrheal disease due to ETEC, particularly among children in low-resource settings. The development of a vaccine against this infection has been hampered by technical constraints, insufficient support for coordination, and a lack of market forces for research and development. Most vaccine development efforts are taking place in the public sector or as research programs within biotechnology companies. ETEC is a longstanding priority and target for vaccine development for the World Health Organization.

Treatment for ETEC infection includes rehydration therapy and antibiotics, although ETEC is frequently resistant to common antibiotics. Improved sanitation is also key. Since the transmission of this bacterium is fecal contamination of food and water supplies, one way to prevent infection is by improving public and private health facilities. Another simple prevention of infection is by drinking factory bottled water—this is especially important for travelers and traveling military—though it may not be feasible in developing countries, which carry the greatest disease burden.

Insufficient data exist, but it is estimated to have caused the death of 34,000 children under the age of five in 2013, and 40,000 deaths in people over five years of age. "Shigella" also causes about 580,000 cases annually among travelers and military personnel from industrialized countries.

An estimated 500,000 cases of shigellosis occur annually in the United States. Infants, the elderly, and the critically ill are susceptible to the most severe symptoms of disease, but all humans are susceptible to some degree. Individuals with acquired immune deficiency syndrome (AIDS) are more frequently infected with "Shigella". Shigellosis is a more common and serious condition in the developing world; fatality rates of shigellosis epidemics in developing countries can be 5–15%.

Orthodox Jewish communities (OJCs) are a known risk group for shigellosis; "Shigella sonnei" is cyclically epidemic in these communities in Israel, with sporadic outbreaks occurring elsewhere in among these communities. "Through phylogenetic and genomic analysis, we showed that strains from outbreaks in OJCs outside of Israel are distinct from strains in the general population and relate to a single multidrug-resistant sublineage of "S. sonnei" that prevails in Israel. Further Bayesian phylogenetic analysis showed that this strain emerged approximately 30 years ago, demonstrating the speed at which antimicrobial drug–resistant pathogens can spread widely through geographically dispersed, but internationally connected, communities."

Currently, no licensed vaccine targeting "Shigella" exists. Several vaccine candidates for "Shigella" are in various stages of development including live attenuated, conjugate, ribosomal, and proteosome vaccines. "Shigella" has been a longstanding World Health Organization target for vaccine development, and sharp declines in age-specific diarrhea/dysentery attack rates for this pathogen indicate that natural immunity does develop following exposure; thus, vaccination to prevent the disease should be feasible. Shigellosis is resistant to many antibiotics used to treat the disease, so vaccination is an important part of the strategy to reduce morbidity and mortality.

Cancers of the stomach are rare and the incidence has been declining worldwide. Stomach cancers usually occur due to fluctuations in acidity level and may present with vague symptoms of abdominal fullness, weight loss and pain. The actual cause of stomach cancer is not known but has been linked to infection with "Helicobacter pylori", pernicious anemia, Menetriere's disease, and nitrogenous preservatives in food.