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").

In the developed world "Campylobacter jejuni" is the primary cause of bacterial gastroenteritis, with half of these cases associated with exposure to poultry. In children, bacteria are the cause in about 15% of cases, with the most common types being "Escherichia coli", "Salmonella", "Shigella", and "Campylobacter" species. If food becomes contaminated with bacteria and remains at room temperature for a period of several hours, the bacteria multiply and increase the risk of infection in those who consume the food. Some foods commonly associated with illness include raw or undercooked meat, poultry, seafood, and eggs; raw sprouts; unpasteurized milk and soft cheeses; and fruit and vegetable juices. In the developing world, especially sub-Saharan Africa and Asia, cholera is a common cause of gastroenteritis. This infection is usually transmitted by contaminated water or food.

Toxigenic "Clostridium difficile" is an important cause of diarrhea that occurs more often in the elderly. Infants can carry these bacteria without developing symptoms. It is a common cause of diarrhea in those who are hospitalized and is frequently associated with antibiotic use. "Staphylococcus aureus" infectious diarrhea may also occur in those who have used antibiotics. Acute "traveler's diarrhea" is usually a type of bacterial gastroenteritis, while the persistent form is usually parasitic. Acid-suppressing medication appears to increase the risk of significant infection after exposure to a number of organisms, including "Clostridium difficile", "Salmonella", and "Campylobacter" species. The risk is greater in those taking proton pump inhibitors than with H2 antagonists.

Rotavirus A, which accounts for more than 90% of rotavirus gastroenteritis in humans, is endemic worldwide. Each year rotavirus causes millions of cases of diarrhoea in developing countries, almost 2 million resulting in hospitalisation and an estimated 453,000 resulting in the death of a child younger than five. This is about 40 per cent of all hospital admissions related to diarrhea in children under five worldwide.

In the United States alone—before initiation of the rotavirus vaccination programme—over 2.7 million cases of rotavirus gastroenteritis occurred annually, 60,000 children were hospitalised and around 37 died from the results of the infection. The major role of rotavirus in causing diarrhoea is not widely recognised within the public health community, particularly in developing countries. Almost every child has been infected with rotavirus by age five. It is the leading single cause of severe diarrhoea among infants and children, being responsible for about 20% of cases, and accounts for 50% of the cases requiring hospitalisation. Rotavirus causes 37% of deaths attributable to diarrhoea and 5% of all deaths in children younger than five. Boys are twice as likely as girls to be admitted to hospital.

Rotavirus infections occur primarily during cool, dry seasons. The number attributable to food contamination is unknown.

Outbreaks of rotavirus A diarrhoea are common among hospitalised infants, young children attending day care centres, and elderly people in nursing homes. An outbreak caused by contaminated municipal water occurred in Colorado in 1981.

During 2005, the largest recorded epidemic of diarrhoea occurred in Nicaragua. This unusually large and severe outbreak was associated with mutations in the rotavirus A genome, possibly helping the virus escape the prevalent immunity in the population. A similar large outbreak occurred in Brazil in 1977.

Rotavirus B, also called adult diarrhoea rotavirus or ADRV, has caused major epidemics of severe diarrhoea affecting thousands of people of all ages in China. These epidemics occurred as a result of sewage contamination of drinking water. Rotavirus B infections also occurred in India in 1998; the causative strain was named CAL. Unlike ADRV, the CAL strain is endemic. To date, epidemics caused by rotavirus B have been confined to mainland China, and surveys indicate a lack of immunity to this species in the United States.

Because improved sanitation does not decrease the prevalence of rotaviral disease, and the rate of hospitalisations remains high, despite the use of oral rehydrating medicines, the primary public health intervention is vaccination. Two rotavirus vaccines against Rotavirus A infection are safe and effective in children: Rotarix by GlaxoSmithKline and RotaTeq by Merck. Both are taken orally and contain attenuated live virus.

Rotavirus vaccines are licensed in more than 100 countries, but only 17 countries have introduced routine rotavirus vaccination. Following the introduction of routine rotavirus vaccination in the US in 2006, the health burden of rotavirus gastroenteritis "rapidly and dramatically reduced" despite lower coverage levels compared to other routine infant immunizations. Clinical trials of the Rotarix rotavirus vaccine in South Africa and Malawi, found that the vaccine significantly reduced severe diarrhoea episodes caused by rotavirus, and that the infection was preventable by vaccination. A 2012 Cochrane review of 41 clinical trials that included 186,263 participants concluded Rotarix and RotaTeq are effective vaccines. Additional rotavirus vaccines are under development. The World Health Organization(WHO) recommends that rotavirus vaccine be included in all national immunisation programmes. The incidence and severity of rotavirus infections has declined significantly in countries that have acted on this recommendation.

The Rotavirus Vaccine Program is a collaboration between PATH, the (WHO), and the U.S. Centers for Disease Control and Prevention, and is funded by the GAVI Alliance. The Program aims to reduce child morbidity and mortality from diarrhoeal disease by making a vaccine against rotavirus available for use in developing countries.

Rotavirus is highly contagious and cannot be treated with antibiotics or other drugs. Because improved sanitation does not decrease the prevalence of rotaviral disease, and the rate of hospitalisations remains high despite the use of oral rehydrating medicines, the primary public health intervention is vaccination. In 1998, a rotavirus vaccine was licensed for use in the United States. Clinical trials in the United States, Finland, and Venezuela had found it to be 80 to 100% effective at preventing severe diarrhoea caused by rotavirus A, and researchers had detected no statistically significant serious adverse effects. The manufacturer, however, withdrew it from the market in 1999, after it was discovered that the vaccine may have contributed to an increased risk for intussusception, a type of bowel obstruction, in one of every 12,000 vaccinated infants. The experience provoked intense debate about the relative risks and benefits of a rotavirus vaccine.

In 2006, two new vaccines against infection were shown to be safe and effective in children, and in 2009, the WHO recommended that rotavirus vaccine be included in all national immunisation programmes.

The incidence and severity of rotavirus infections has declined significantly in countries that have acted on this recommendation. A 2014 review of available clinical trial data from countries routinely using rotavirus vaccines in their national immunisation programs found that rotavirus vaccines have reduced rotavirus hospitalisations by 49–92 percent and all cause diarrhoea hospitalisations by 17–55 percent. In Mexico, which in 2006 was among the first countries in the world to introduce rotavirus vaccine, diarrhoeal disease death rates dropped during the 2009 rotavirus season by more than 65 percent among children age two and under. In Nicaragua, which in 2006 became the first developing country to introduce a rotavirus vaccine, severe rotavirus infections were reduced by 40 percent and emergency room visits by a half. In the United States, rotavirus vaccination since 2006 has led to drops in rotavirus-related hospitalisations by as much as 86 percent. The vaccines may also have prevented illness in non-vaccinated children by limiting the number of circulating infections. In developing countries in Africa and Asia, where the majority of rotavirus deaths occur, a large number of safety and efficacy trials as well as recent post-introduction impact and effectiveness studies of Rotarix and RotaTeq have found that vaccines dramatically reduced severe disease among infants. In September 2013, the vaccine was offered to all children in the UK, aged between two and three months, and it is expected to halve the cases of severe infection and reduce the number of children admitted to hospital because of the infection by 70 percent. In Europe, hospitalisation rates following infection by rotavirus have decreased by 65% to 84% following the introduction of the vaccine. Globally, vaccination has reduced hospital admissions and emergency department visits by a median of 67%.

Rotavirus vaccines are licensed in over 100 countries, and more than 80 countries have introduced routine rotavirus vaccination, almost half with the support of Gavi, the Vaccine Alliance. To make rotavirus vaccines available, accessible, and affordable in all countries—particularly low- and middle-income countries in Africa and Asia where the majority of rotavirus deaths occur, PATH (formerly Program for Appropriate Technology in Health), the WHO, the U.S. Centers for Disease Control and Prevention, and Gavi have partnered with research institutions and governments to generate and disseminate evidence, lower prices, and accelerate introduction.

Rotavirus A, which accounts for more than 90% of rotavirus gastroenteritis in humans, is endemic worldwide. Each year rotavirus causes millions of cases of diarrhoea in developing countries, almost 2 million of which result in hospitalisation. In 2013, an estimated 215,000 children younger than five died from rotavirus, 90 percent of whom were in developing countries. Almost every child has been infected with rotavirus by age five. Rotavirus is the leading single cause of severe diarrhoea among infants and children, is responsible for about a third of the cases requiring hospitalisation, and causes 37% of deaths attributable to diarrhoea and 5% of all deaths in children younger than five. Boys are twice as likely as girls to be admitted to hospital for rotavirus.

In the pre-vaccination era, rotavirus infections occurred primarily during cool, dry seasons. The number attributable to food contamination is unknown.

Outbreaks of rotavirus A diarrhoea are common among hospitalised infants, young children attending day care centres, and elderly people in nursing homes. An outbreak caused by contaminated municipal water occurred in Colorado in 1981.

During 2005, the largest recorded epidemic of diarrhoea occurred in Nicaragua. This unusually large and severe outbreak was associated with mutations in the rotavirus A genome, possibly helping the virus escape the prevalent immunity in the population. A similar large outbreak occurred in Brazil in 1977.

Rotavirus B, also called adult diarrhoea rotavirus or ADRV, has caused major epidemics of severe diarrhoea affecting thousands of people of all ages in China. These epidemics occurred as a result of sewage contamination of drinking water. Rotavirus B infections also occurred in India in 1998; the causative strain was named CAL. Unlike ADRV, the CAL strain is endemic. To date, epidemics caused by rotavirus B have been confined to mainland China, and surveys indicate a lack of immunity to this species in the United States.

Rotavirus C has been associated with rare and sporadic cases of diarrhoea in children, and small outbreaks have occurred in families.

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."

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

The best prevention against viral pneumonia is vaccination against influenza, adenovirus, chickenpox, herpes zoster, measles, and rubella.

Adenovirus can cause severe necrotizing pneumonia in which all or part of a lung has increased translucency radiographically, which is called Swyer-James Syndrome. Severe adenovirus pneumonia also may result in bronchiolitis obliterans, a subacute inflammatory process in which the small airways are replaced by scar tissue, resulting in a reduction in lung volume and lung compliance.

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

Numerous factors have been suggested and linked to a higher risk of acquiring the infection, inclusive of malnutrition, vitamin A deficiency, absence of breastfeeding during the early stages of life, environmental pollution and overcrowding.

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.

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.

Mortality caused by HPIVs in developed regions of the world remains rare. Where mortality has occurred, it is principally in the three core risk groups (very young, elderly and immuno-compromised). Long term changes can however be associated with airway remodelling and are believed to be a significant cause of morbidity. The exact associations between HPIVs and diseases such as chronic obstructive pulmonary disease (COPD) are still being investigated.

In developing regions of the world, the highest risk group in terms of mortality remains pre-school children. Mortality may be as a consequence of primary viral infection or secondary problems such as bacterial infection. Predispositions, such as malnutrition and other deficiencies may further elevate the chances of mortality associated with infection.

Overall, LRI's cause approximately 25–30% of total deaths in pre-school children in the developing world. HPIVs is believed to be associated with 10% of all LRI cases, thus remaining a significant cause of mortality.

Gastroenteritis can be caused by viral, bacterial, or parasitic infections. Common routes of infection include:

- Food

- Contaminated water

- Contact with an infected person

- Unwashed hands

Fifty to seventy percent of cases of gastroenteritis in adults are caused by noroviruses (genus Norovirus, family Caliciviridae). This virus is highly contagious and spreads rapidly. Norovirus is the most common cause of gastroenteritis in the United States.

Viral pneumonia occurs in about 200 million people a year which includes about 100 million children and 100 million adults.

The common routes of transmission for the disease-causing bacteria are fecal-oral, person-to-person sexual contact, ingestion of contaminated food (generally unpasteurized (raw) milk and undercooked or poorly handled poultry), and waterborne (i.e., through contaminated drinking water). Contact with contaminated poultry, livestock, or household pets, especially puppies, can also cause disease.

Animals farmed for meat are the main source of campylobacteriosis. A study published in PLoS Genetics (September 26, 2008) by researchers from Lancashire, England, and Chicago, Illinois, found that 97 percent of campylobacteriosis cases sampled in Lancashire were caused by bacteria typically found in chicken and livestock. In 57 percent of cases, the bacteria could be traced to chicken, and in 35 percent to cattle. Wild animal and environmental sources were accountable for just three percent of disease.

The infectious dose is 1000–10,000 bacteria (although ten to five hundred bacteria can be enough to infect humans). "Campylobacter" species are sensitive to hydrochloric acid in the stomach, and acid reduction treatment can reduce the amount of needed to cause disease.

Exposure to bacteria is often more common during travelling, and therefore campylobacteriosis is a common form of travelers' diarrhea.

Although epidemiologic characteristics of the adenoviruses vary by type, all are transmitted by direct contact, fecal-oral transmission, and occasionally waterborne transmission. Some types are capable of establishing persistent asymptomatic infections in tonsils, adenoids, and intestines of infected hosts, and shedding can occur for months or years. Some adenoviruses (e.g., serotypes 1, 2, 5, and 6) have been shown to be endemic in parts of the world where they have been studied, and infection is usually acquired during childhood. Other types cause sporadic infection and occasional outbreaks; for example, epidemic keratoconjunctivitis is associated with adenovirus serotypes 8, 19, and 37. Epidemics of febrile disease with conjunctivitis are associated with waterborne transmission of some adenovirus types, often centering on inadequately chlorinated swimming pools and small lakes. ARD is most often associated with adenovirus types 4 and 7 in the United States. Enteric adenoviruses 40 and 41 cause gastroenteritis, usually in children. For some adenovirus serotypes, the clinical spectrum of disease associated with infection varies depending on the site of infection; for example, infection with adenovirus 7 acquired by inhalation is associated with severe lower respiratory tract disease, whereas oral transmission of the virus typically causes no or mild disease. Outbreaks of adenovirus-associated respiratory disease have been more common in the late winter, spring, and early summer; however, adenovirus infections can occur throughout the year.

"Ad14 (for adenovirus serotype 14), has caused at least 140 illnesses in New York, Oregon, Texas and Washington, according to a report from the Centers for Disease Control and Prevention. The illness made headlines in Texas in September 2007, when a so-called "boot camp flu" sickened hundreds at Lackland Air Force Base in San Antonio. A 19-year-old trainee died."

Several adenoviruses, including Ad5, Ad9, Ad31, Ad36, Ad37, and SMAM1, have at least some evidence of causation of obesity in animals, adipogenesis in cells, and/or association with human obesity. To date, the most thorough investigations have been conducted for adenovirus serotype 36 (Adv36).

Dysentery may also be caused by shigellosis, an infection by bacteria of the genus "Shigella", and is then known as bacillary dysentery (or Marlow syndrome). The term "bacillary dysentery" etymologically might seem to refer to any dysentery caused by any bacilliform bacteria, but its meaning is restricted by convention to "Shigella" dysentery.

Campylobacteriosis is usually self-limited without any mortality (assuming proper hydration is maintained). However, there are several possible complications.

Treatment is symptomatic and aims to prevent dehydration in young pigs, using products such as electrolyte and energy supplements. Good biosecurity protocols such as adequate quarantine, isolation of cases, and disinfection help prevent entry or spread of the disease in the herd. In Canada, the Canadian Swine Health Board developed detailed protocols on how to adequately disinfect transportation vehicles for live hogs and ensure the quality of the disinfecttion protocol.

Many cases of croup have been prevented by immunization for influenza and diphtheria. At one time, croup referred to a diphtherial disease, but with vaccination, diphtheria is now rare in the developed world.

Influenza's effects are much more severe and last longer than those of the common cold. Most people will recover completely in about one to two weeks, but others will develop life-threatening complications (such as pneumonia). Thus, influenza can be deadly, especially for the weak, young and old, or chronically ill. People with a weak immune system, such as people with advanced HIV infection or transplant patients (whose immune systems are medically suppressed to prevent transplant organ rejection), suffer from particularly severe disease. Pregnant women and young children are also at a high risk for complications.

The flu can worsen chronic health problems. People with emphysema, chronic bronchitis or asthma may experience shortness of breath while they have the flu, and influenza may cause worsening of coronary heart disease or congestive heart failure. Smoking is another risk factor associated with more serious disease and increased mortality from influenza.

According to the World Health Organization: "Every winter, tens of millions of people get the flu. Most are only ill and out of work for a week, yet the elderly are at a higher risk of death from the illness. We know the worldwide death toll exceeds a few hundred thousand people a year, but even in developed countries the numbers are uncertain, because medical authorities don't usually verify who actually died of influenza and who died of a flu-like illness." Even healthy people can be affected, and serious problems from influenza can happen at any age. People over 65 years old, pregnant women, very young children and people of any age with chronic medical conditions are more likely to get complications from influenza, such as pneumonia, bronchitis, sinus, and ear infections.

In some cases, an autoimmune response to an influenza infection may contribute to the development of Guillain–Barré syndrome. However, as many other infections can increase the risk of this disease, influenza may only be an important cause during epidemics. This syndrome has been believed to also be a rare side effect of influenza vaccines. One review gives an incidence of about one case per million vaccinations. Getting infected by influenza itself increases both the risk of death (up to 1 in 10,000) and increases the risk of developing GBS to a much higher level than the highest level of suspected vaccine involvement (approx. 10 times higher by recent estimates).