Escherichia coli O104:H21 is a rare serotype of Escherichia coli, a species of bacteria that lives in the lower intestines of mammals. Although there are many serotypes of E. coli, when in animals, there are benefits or do not cause disease. Some serotypes of E. coli have been recognized as pathogenic to humans, e.g. E. coli O157:H7, E. coli O121 and E. coli O104:H21.