Enterohemorrhagic Escherichia coli (EHEC) is a subset of pathogenic E. coli that can cause diarrhea or hemorrhagic colitis in humans. Hemorrhagic colitis occasionally progresses to hemolytic uremic syndrome (HUS), an important cause of acute renal failure in children and morbidity and mortality in adults. Enterohemorrhagic E. coli O157:H7 (EHEC O157:H7) has been known to cause these syndromes since the 1980s, but clinical cases and outbreaks caused by members of other EHEC serogroups are increasingly recognized. In some areas, non-O157 EHEC may account for a greater number of cases than EHEC O157:H7. In 2011, an unusual enteroaggregative E. coli (EAEC) with the serotype O104:H4 was responsible for a severe outbreak of hemorrhagic colitis and HUS in Europe. What all of the HUSassociated E. coli seem to have in common is the ability to produce verotoxins, together with the ability to bind to and colonize human intestines. Because verotoxin genes can be transmitted between bacteria, additional E. coli pathotypes associated with HUS could also be discovered.

Ruminants, particularly cattle and sheep, seem to be the maintenance hosts for EHEC O157:H7 and many other verotoxin-producing E. coli. Some, but not all, individual animals carry these organisms in the intestinal tract, and shed them in the feces. Members of other animal species are also infected occasionally. Most infected animals do not develop any clinical signs, although members of some non-O157 serogroups may cause enteric disease in young animals, and EHEC O153 has been linked to a disease that resembles HUS in rabbits. Humans acquire EHEC by direct contact with animal carriers, their feces, infected people, and contaminated soil or water, or via the ingestion of underdone meat, other animal products, contaminated vegetables and fruit, and other foods. The infectious dose for people is very low, which increases the risk of disease. Animals do not seem to be reservoirs for enteroaggregative, verotoxin-producing E. coli, which are probably maintained in humans, but can also be acquired in food.