Escherichia coli are a diverse group of bacteria found in the environment and in the intestines of humans and animals. Most strains of E coli are harmless, while others can cause a variety of infections including diarrhea. The 6 pathotypes of E coli that cause diarrhea are characterized by the kind of clinical symptoms they produce.

Enterotoxigenic E coli (ETEC) – The leading cause of traveler's diarrhea worldwide. Transmitted by food and/or water contaminated with animal or human feces. Especially prevalent in children from developing countries and also a cause of traveler's diarrhea. This causes profuse watery diarrhea and abdominal cramping. Other associated symptoms include fever, nausea, vomiting, chills, loss of appetite, headache, muscle aches, and bloating. Symptoms develop 1-3 days after exposure and last for 3-4 days.

Enteropathogenic E coli (EPEC) – These strains have a characteristic attaching mechanism that allows them to adhere to enterocytes of the gut. This attachment causes induction of signal transduction pathways that alter water and electrolyte secretion, leading to diarrhea. This mostly affects children under 6 months of age in developing countries. Associated symptoms include vomiting. Dehydration and malnutrition are potentially deadly side effects, especially in children. Adults are less likely to be symptomatic.

Enteroinvasive E coli (EIEC) – Uncommon, related to Shigella. Usually begins with watery diarrhea, which may progress to bloody diarrhea.

Shiga toxin-producing E coli (STEC), also called enterohemorrhagic E coli (EHEC) – Included in this group is E coli O157, which is known to cause large outbreaks. STEC is found in the gut of ruminant animals such as cattle, goats, sheep, and deer. Infections occur when humans are exposed to human or animal feces with STEC. High-risk foods include unpasteurized milk, unpasteurized apple cider, and soft cheeses made with raw milk. Adults and children are equally affected, with more severe disease seen in young children and the elderly. Symptoms include abdominal cramping, diarrhea (often bloody), vomiting, and fever. There is an incubation period of 3-4 days after exposure. The duration of disease is typically 5-7 days. A serious complication seen in 5%-10% of affected individuals is hemolytic uremic syndrome (HUS), which can cause irreversible kidney damage.

Enteroaggregative E coli (EAEC) – This strain expresses adherence fimbriae and a flagellin, which causes the release of interleukin-8. Also associated with a cytotoxin that causes mucosal destruction. It is a common cause of acute diarrheal illness in adults and children in the developing world. Sporadic outbreaks have occurred in developed countries. Adults with human immunodeficiency virus (HIV) may be particularly susceptible. 

Diffusely adherent E coli (DAEC) – A cause of diarrhea in children, less so in adults. Strains express Afa/Dr adhesins that allow binding to gut enterocytes. Children are thought to be more susceptible than adults due to lack of maturity of the intestinal epithelial barrier.