Escherichia coli (E. coli) is a germ that occurs naturally in the gut of mammals and birds, as well as in the human intestinal flora. However, certain E. coli types can cause severe diarrhea in humans. These virulent E. coli types include Shiga toxin-producing E. coli (STEC), also known as Verotoxin-producing E. coli (VTEC).