Bacterial enteropathogens cause about 80% of cases. Viruses and protozoans account for most of the rest.The most common causative agent isolated in countries surveyed has been enterotoxigenic Escherichia coli (ETEC). Enteroaggregative E. coli is increasingly recognized. Shigella spp. and Salmonella spp. are other common bacterial pathogens. Campylobacter, Yersinia, Aeromonas, and Plesiomonas spp. are less frequently found. Mechanisms of action vary: some bacteria release toxins which bind to the intestinal wall and cause diarrhea; others damage the intestines themselves by their direct presence.Brachyspira pilosicoli pathogen also appears to be responsible for many chronic intermittent watery diarrhea and is only diagnosed through colonic biopsies and microscopic discovery of a false brush border  on H&E or Warthin silver stain: its brush-border is stronger and longer that Brachyspira aalborgis brush-border. It is unfortunately often not diagnosed as coproculture does not allow growth and 16S PCR panel primers do not match Brachyspira sequences.While viruses are associated with less than 20% of adult cases of travelers diarrhea, they may be responsible for nearly 70% of cases in infants and children. Diarrhea due to viral agents is unaffected by antibiotic therapy, but is usually self-limited. Protozoans such as Giardia lamblia, Cryptosporidium and Cyclospora cayetanensis can also cause diarrhea. Pathogens commonly implicated in travelers diarrhea appear in the table in this section.A subtype of travelers diarrhea afflicting hikers and campers, sometimes known as wilderness diarrhea, may have a somewhat different frequency of distribution of pathogens. Risk factors
The primary source of infection is ingestion of fecally contaminated food or water.
Unpasteurized milk, dairy products, mayonnaise, and pastry icing are associated with increased risk for TD, as are foods and beverages purchased from street vendors and other establishments where unhygienic conditions may be present. Water
Although safe bottled water is now widely available in most remote destinations, travelers can treat their own water if necessary, or as an extra precaution. Techniques include boiling, filtering, chemical treatment, and ultraviolet light; boiling is by far the most effective of these methods. Boiling rapidly kills all active bacteria, viruses, and protozoa. Prolonged boiling is usually unnecessary; most microorganisms are killed within seconds at water temperature above 55–70 °C (130–160 °F). The second-most effective method is to combine filtration and chemical disinfection. Filters eliminate most bacteria and protozoa, but not viruses. Chemical treatment with halogens—chlorine bleach, tincture of iodine, or commercial tablets—have low-to-moderate effectiveness against protozoa such as Giardia, but work well against bacteria and viruses. UV light is effective against both viruses and cellular organisms, but only works in clear water, and it is ineffective unless manufacturers instructions are carefully followed for maximum water depth/distance from UV source, and for dose/exposure time. Other claimed advantages include short treatment time, elimination of the need for boiling, no taste alteration, and decreased long-term cost compared with bottled water. The effectiveness of UV devices is reduced when water is muddy or turbid; as UV is a type of light, any suspended particles create shadows that hide microorganisms from UV exposure. Medications
Bismuth subsalicylate four times daily reduces rates of travelers diarrhea.