E. coli are coliform bacteria that live in the intestines of humans and animals. Most strains of E. coli are harmless, but some are known to make toxins that can cause diarrhea and illness. One E. coli strain called O157:H7 can cause severe diarrhea and kidney damage.Return to Top.ost strains of E. coli are harmless, but some are known to make toxins that can cause diarrhea and illness. One E. coli strain called O157:H7 can cause severe diarrhea and kidney damage.