Although most strains of E. coli are harmless and exist as part of the normal GI flora, infection usually results from certain nonindigenous strains. For example, noninvasive diarrhea results from two toxins produced by strains called enterotoxic or enteropathogenic E. coli. Enteropathogenic E. coli serotype O157:H7 is the most well-known strain in the United States. Strains that produce Shiga toxins, known as Shiga toxin–producing E. coli, can result in hemorrhagic colitis (a form of renal failure) and hemolytic-uremic syndrome (HUS). HUS is a serious condition that affects approximately 10% of people who are infected with E. coli O157:H7 or other Shiga toxin–producing E. coli; of those who are infected, 50% will require dialysis and 5% will die. These toxins interact with intestinal juices and promote excessive loss of chloride and water. In the invasive form, E. coli directly invades the intestinal mucosa without producing enterotoxins, thereby causing local irritation, inflammation, and diarrhea. Normal strains can cause infection in immunocompromised patients.

Transmission can occur directly from an infected person or indirectly by ingestion of contaminated food or water or contact with contaminated utensils. Cattle and other ruminants are an important reservoir in the United States, with the mode of transmission being the ingestion of food or water contaminated with the infected animal’s feces. The incubation period can be as short as 12 hours or as long as 4 days.

The incidence of E. coli infection is highest among travelers returning from other countries, particularly Mexico, Southeast Asia, and South America. E. coli infection also induces other diseases, especially in people whose resistance is low. Outbreaks of E. coli O157:H7 have been associated with undercooked hamburger, raw cookie dough, unpasteurized milk, and produce such as onions, melons, and spinach. Waterborne transmission can occur from contaminated drinking water and recreational waters. The infection can also be transmitted following contact with animals at petting zoos.