E. coli is a normal inhabitant of the intestines of all animals, including humans. When aerobic culture methods are used, E. coli is the dominant species found in feces. Normally E. coli serves a useful function in the body by suppressing the growth of harmful bacterial species and by synthesizing appreciable amounts of vitamins. A minority of E. coli strains are capable of causing human illness by several different mechanisms. Occasionally vomiting occurs.
Fever is either low-grade or absent.
The illness is usually self-limited and lasts for an average of 8 days. Some individuals exhibit watery diarrhea only. Alternatively, the stools can be tested directly for the presence of verotoxin. Confirmation can be obtained by isolation of E. coli of the same serotype from the incriminated food. Raw milk was the vehicle in a school outbreak in Canada.