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.

The World Health Organization recommends the following:

- Food should be properly cooked and hot when served.

- Consume only pasteurized or boiled milk and milk products, never raw milk products.

- Make sure that ice is from safe water.

- If you are not sure of the safety of drinking water, boil it, or disinfect it with chemical disinfectant.

- Wash hands thoroughly and frequently with soap, especially after using the toilet and after contact with pets and farm animals.

- Wash fruits and vegetables thoroughly, especially if they are to be eaten raw. Peel fruits and vegetables whenever possible.

- Food handlers, professionals and at home, should observe hygienic rules during food preparation.

- Professional food handlers should immediately report to their employer any fever, diarrhea, vomiting or visible infected skin lesions.

Treatment for gastroenteritis due to "Y. enterocolitica" is not needed in the majority of cases. Severe infections with systemic involvement (sepsis or bacteremia) often requires aggressive antibiotic therapy; the drugs of choice are doxycycline and an aminoglycoside. Alternatives include cefotaxime, fluoroquinolones, and co-trimoxazole.

Yersiniosis is usually self-limiting and does not require treatment. For severe infections (sepsis, focal infection) especially if associated with immunosuppression, the recommended regimen includes doxycycline in combination with an aminoglycoside. Other antibiotics active against "Y. enterocolitica" include trimethoprim-sulfamethoxasole, fluoroquinolones, ceftriaxone, and chloramphenicol. "Y. enterocolitica" is usually resistant to penicillin G, ampicillin, and cephalotin due to beta-lactamase production.

"Y. enterocolitica" infections are sometimes followed by chronic inflammatory diseases such as arthritis, erythema nodosum, and reactive arthritis. This is most likely because of some immune-mediated mechanism.

"Y. enterocolitica" seems to be associated with autoimmune Graves-Basedow thyroiditis.

Whilst indirect evidence exists, direct causative evidence is limited,

and "Y. enterocolitica" is probably not a major cause of this disease, but may contribute to the development of thyroid autoimmunity arising for other reasons in genetically susceptible individuals.

"Y. enterocolitica" infection has also been suggested to not be the cause of autoimmune thyroid disease, but rather is only an associated condition, with both having a shared inherited susceptibility.

More recently, the role for "Y. enterocolitica" has been disputed.

Yersiniosis is an infectious disease caused by a bacterium of the genus "Yersinia". In the United States, most yersiniosis infections among humans are caused by "Yersinia enterocolitica". The infection by "Y. enterocolitica" is also known as pseudotuberculosis. Yersiniosis is mentioned as a specific zoonotic disease to prevent outbreaks in European Council Directive 92/117/EEC.

Infection with " Y . enterocolitica" occurs most often in young children. The infection is thought to be contracted through the consumption of undercooked meat products, unpasteurized milk, or water contaminated by the bacteria. It has been also sometimes associated with handling raw chitterlings.

Another bacterium of the same genus, "Yersinia pestis", is the cause of Plague.

The U.S. Centers for Disease Control and Prevention (CDC) publishes a journal "Emerging Infectious Diseases" that identifies the following factors contributing to disease emergence:

- Microbial adaption; e.g. genetic drift and genetic shift in Influenza A

- Changing human susceptibility; e.g. mass immunocompromisation with HIV/AIDS

- Climate and weather; e.g. diseases with zoonotic vectors such as West Nile Disease (transmitted by mosquitoes) are moving further from the tropics as the climate warms

- Change in human demographics and trade; e.g. rapid travel enabled SARS to rapidly propagate around the globe

- Economic development; e.g. use of antibiotics to increase meat yield of farmed cows leads to antibiotic resistance

- Breakdown of public health; e.g. the current situation in Zimbabwe

- Poverty and social inequality; e.g. tuberculosis is primarily a problem in low-income areas

- War and famine

- Bioterrorism; e.g. 2001 Anthrax attacks

- Dam and irrigation system construction; e.g. malaria and other mosquito borne diseases

Specific types of enterocolitis include:

- necrotizing enterocolitis (most common in premature infants)

- pseudomembranous enterocolitis (also called "Pseudomembranous colitis")

Methicillin-resistant Staphylococcus aureus (MRSA) evolved from Methicillin-susceptible Staphylococcus aureus (MSSA) otherwise known as common "S. aureus". Many people are natural carriers of "S. aureus", without being affected in any way. MSSA was treatable with the antibiotic methicillin until it acquired the gene for antibiotic resistance. Though genetic mapping of various strains of MRSA, scientists have found that MSSA acquired the mecA gene in the 1960s, which accounts for its pathogenicity, before this it had a predominantly commensal relationship with humans. It is theorized that when this "S. aureus" strain that had acquired the mecA gene was introduced into hospitals, it came into contact with other hospital bacteria that had already been exposed to high levels of antibiotics. When exposed to such high levels of antibiotics, the hospital bacteria suddenly found themselves in an environment that had a high level of selection for antibiotic resistance, and thus resistance to multiple antibiotics formed within these hospital populations. When "S. aureus" came into contact with these populations, the multiple genes that code for antibiotic resistance to different drugs were then acquired by MRSA, making it nearly impossible to control. It is thought that MSSA acquired the resistance gene through the horizontal gene transfer, a method in which genetic information can be passed within a generation, and spread rapidly through its own population as was illustrated in multiple studies. Horizontal gene transfer speeds the process of genetic transfer since there is no need to wait an entire generation time for gene to be passed on. Since most antibiotics do not work on MRSA, physicians have to turn to alternative methods based in Darwinian medicine. However prevention is the most preferred method of avoiding antibiotic resistance. By reducing unnecessary antibiotic use in human and animal populations, antibiotics resistance can be slowed.

Enterocolitis or coloenteritis is an inflammation of the digestive tract, involving enteritis of the small intestine and colitis of the colon. It may be caused by various infections, with bacteria, viruses, fungi, parasites, or other causes. Common clinical manifestations of enterocolitis are frequent diarrheal defecations, with or without nausea, vomiting, abdominal pain, fever, chills, alteration of general condition. General manifestations are given by the dissemination of the infectious agent or its toxins throughout the body, or – most frequently – by significant losses of water and minerals, the consequence of diarrhea and vomiting.

Among the causal agents of acute enterocolitis are:

- bacteria: "Salmonella", "Shigella", "Escherichia coli", "Campylobacter" etc.;

- viruses: enteroviruses, rotaviruses, Norwalk virus, adenoviruses;

- fungi: candidiasis, especially in immunosuppressed patients or who have previously received prolonged antibiotic treatment;

- parasites: "Giardia lamblia" (with high frequency of infestation in the population, but not always with clinical manifestations), "Balantidium coli", "Blastocystis homnis", "Cryptosporidium" (diarrhea in people with immunosuppression), "Entamoeba histolytica" (produces the amebian dysentery, common in tropical areas).

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.

Staphylococcal enteritis is an inflammation that is usually caused by eating or drinking substances contaminated with staph enterotoxin. The toxin, not the bacterium, settles in the small intestine and causes inflammation and swelling. This in turn can cause abdominal pain, cramping, dehydration, diarrhea and fever.

"Staphylococcus aureus" is a Gram-positive, facultative anaerobe, coccal (round shaped) bacteria that appears in grape-like clusters that can thrive in high salt and low water activity habitats. "S. aureus" bacteria can live on the skin which is one of the primary modes of transmission. "S. aureus" can cause a range of illnesses from minor skin infections to Staphylococcus aureus food poisoning enteritis. Since humans are the primary source, cross-contamination is the most common way the microorganism is introduced into foods. Foods at high risks are those prepared in large quantities.

Staphylococcus aureus is a true food poisoning organism. It produces a heat stable enterotoxin when allowed to grow for several hours in foods such as cream-filled baked goods, poultry meat, gravies, eggs, meat salads, puddings and vegetables. It is important to note that the toxins may be present in dangerous amounts in foods that have no signs of spoilage, such as a bad smell, any off color, odor, or textural or flavor change.

Enteritis is the inflammation of the small intestine. It is generally caused by eating or drinking substances that are contaminated with bacteria or viruses. The bacterium and/or toxin settles in the small intestine and cause inflammation and swelling. This in turn can cause abdominal pain, cramping, diarrhea, fever, and dehydration. There are other types of enteritis, the types include: bacterial gastroenteritis, "Campylobacter" enteritis, "E. coli" enteritis, radiation enteritis, "Salmonella" enteritis and "Shigella" enteritis.

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

"S. aureus" is an enterotoxin producer. Enterotoxins are chromosomally encoded exotoxins that are produced and secreted from several bacterial organisms. It is a heat stable toxin and is resistant to digestive protease. It is the ingestion of the toxin that causes the inflammation and swelling of the intestine.

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.

Yersinia pseudotuberculosis is a Gram-negative bacterium that causes Far East scarlet-like fever in humans, who occasionally get infected zoonotically, most often through the food-borne route. Animals are also infected by "Y. pseudotuberculosis". The bacterium is urease positive.

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.

In animals, "Y. pseudotuberculosis" can cause tuberculosis-like symptoms, including localized tissue necrosis and granulomas in the spleen, liver, and lymph nodes.

In humans, symptoms of Far East scarlet-like fever are similar to those of infection with "Yersinia enterocolitica" (fever and right-sided abdominal pain), except that the diarrheal component is often absent, which sometimes makes the resulting condition difficult to diagnose. "Y. pseudotuberculosis" infections can mimic appendicitis, especially in children and younger adults, and, in rare cases, the disease may cause skin complaints (erythema nodosum), joint stiffness and pain (reactive arthritis), or spread of bacteria to the blood (bacteremia).

Far East scarlet-like fever usually becomes apparent five to 10 days after exposure and typically lasts one to three weeks without treatment. In complex cases or those involving immunocompromised patients, antibiotics may be necessary for resolution; ampicillin, aminoglycosides, tetracycline, chloramphenicol, or a cephalosporin may all be effective.

The recently described syndrome "Izumi-fever" has been linked to infection with "Y. pseudotuberculosis".

The symptoms of fever and abdominal pain mimicking appendicitis (actually from mesenteric lymphadenitis) associated with "Y. pseudotuberculosis" infection are not typical of the diarrhea and vomiting from classical food poisoning incidents. Although "Y. pseudotuberculosis" is usually only able to colonize hosts by peripheral routes and cause serious disease in immunocompromised individuals, if this bacterium gains access to the blood stream, it has an LD comparable to "Y. pestis" at only 10 CFU.

Some fish species serve as vectors for the disease and have subsequently spread the pathogen to other parts of the world. An example is the fathead minnow ("Pimephales promelas") which is responsible for the spread of redmouth disease to trout in Europe. Other vectors include the goldfish ("Carassius auratus"), Atlantic and Pacific salmon ("Salmo salar"), the emerald shiner ("Notropis atherinoides"), and farmed whitefish ("Coregonus" spp.). Infections have also occurred in farmed turbot ("Scophthalmus maximus"), seabass ("Dicentrarchus labrax"), and seabream ("Sparus auratus"). It can now be found in North and South America, Africa, Asia, and Australia, as well as Europe.

In Germany, 90% of cases of infectious enteritis are caused by four pathogens, Norovirus, Rotavirus, "Campylobacter" and "Salmonella". Other common causes of infectious enteritis include bacteria such as "Shigella" and "E. coli," as well as viruses such as adenovirus, astrovirus and calicivirus. Other less common pathogens include "Bacillus cereus, Clostridium perfringens, Clostridium difficile" and "Staphylococcus aureus".

"Campylobacter jejuni" is one of the most common sources of infectious enteritis, and the most common bacterial pathogen found in 2 year old and smaller children with diarrhoea. It has been linked to consumption of contaminated water and food, most commonly poultry and milk. The disease tends to be less severe in developing countries, due to the constant exposure which people have with the antigen in the environment, leading to early development of antibodies.

Rotavirus is responsible for infecting 140 million people and causing 1 million deaths each year, mostly in children younger than 5 years. This makes it the most common cause of severe childhood diarrhoea and diarrhea-related deaths in the world. It selectively targets mature enterocytes in the small intestine, causing malabsorption, as well as inducing secretion of water. It has also been observed to cause villus ischemia, and increase intestinal motility. The net result of these changes is induced diarrhoea.

Enteritis necroticans is an often fatal illness, caused by β-toxin of "Clostridium perfringens". This causes inflammation and segments of necrosis throughout the gastrointestinal tract. It is most common in developing countries, however has also been documented in post-World War II Germany. Risk factors for enteritis necroticans include decreased trypsin activity, which prevent intestinal degradation of the toxin, and reduced intestinal motility, which increases likelihood of toxin accumulation.

Enteric redmouth disease, or simply redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen "Yersinia ruckeri". It is primarily found in rainbow trout ("Oncorhynchus mykiss") and other cultured salmonids. The disease is characterized by subcutaneous hemorrhaging of the mouth, fins, and eyes. It is most commonly seen in fish farms with poor water quality. Redmouth disease was first discovered in Idaho rainbow trout in the 1950s. The disease does not infect humans.

Amphistomiasis in farm and wild mammals is due to infection of paramphistomes, such as the species of "Paramphistomum", "Calicophoron", "Cotylophoron", "Pseudophisthodiscus", etc. These are essentially rumen flukes, of which "Paramphistomum cervi" is the most notorious in terms of prevalence and pathogenicity. Infection occurs through ingestion of contaminated vegetables and raw meat, in which the viable infective metacercaria are deposited from snails, which are the intermediate hosts. The immature flukes are responsible for destroying the mucosal walls of the alimentary tract on their way to growing into adults. It is by this fervent tissue obliteration that the clinical symptoms are manifested. The adult flukes, on the other hand, are quite harmless, as they merely prepare for reproduction.

The zoonotic infection in human is caused by "G. discoides" and "W. watsoni" which are essentially intestinal flukes. The disease due to "G. discoides" is more specifically termed gastrodiscoidiasis. In their natural hosts such as pigs and monkeys, their infection in asymptomatic, but human infection is prevalent, by which they cause serious health problems, characterised by diarrhoea, fever, abdominal pain, colic, and an increased mucous production. In extreme situations such as in Assam, India, a number of mortality among children is attributed to this disease.

Crohn's disease – also known as regional enteritis, it can occur along any surface of the gastrointestinal tract. In 40% of cases it is limited to the small intestine.

Coeliac disease – caused by an autoimmune reaction to gluten by genetically predisposed individuals.

Eosinophilic enteropathy – a condition where eosinophils build up in the gastrointestinal tract and blood vessels, leading to polyp formation, necrosis, inflammation and ulcers. It is most commonly seen in patients with a history of atopy, however is overall relatively uncommon.

Brucellosis in humans is usually associated with the consumption of unpasteurized milk and soft cheeses made from the milk of infected animals, primarily goats, infected with "Brucella melitensis" and with occupational exposure of laboratory workers, veterinarians, and slaughterhouse workers. Some vaccines used in livestock, most notably "B. abortus" strain 19, also cause disease in humans if accidentally injected. Brucellosis induces inconstant fevers, miscarriage, sweating, weakness, anaemia, headaches, depression, and muscular and bodily pain. The other strains, "B. suis" and "B. canis", cause infection in pigs and dogs, respectively.

Causing agents may include

- viruses : reovirus (often considered as unique cause), adenoviruses, enteroviruses, rotaviruses, parvoviruses.

- bacteria like Escherichia coli, Proteus mirabilis, Enterococcus faecium, Staphylococcus cohnii, Clostridium perfringens, Bacteroides fragilis and Bacillus licheniformis, often isolated in affected birds.