Escherichia coli (E. coli) are commonly found bacteria that normally live in the intestines of humans and animals. While most E. coli strains are harmless, some can cause illness such as infant diarrhea, urinary tract infections, and foodborne illness. E. coli have a highly plastic genome which allows different pathotypes to emerge, including the deadly E. coli O104:H4 outbreak linked to sprouts. Some commensal E. coli strains produce a toxin called colibactin that can cause DNA damage in host cells and may promote colon cancer. Further research is needed to better understand the factors that allow some E. coli to switch between commensalism and pathogenicity.