Escherichia coli comes in many different forms — far too many, in fact, to list here. Most are completely harmless. E. coli occurs naturally in human beings and is known to aid our digestive system. The most dangerous strains, those that cause the well-known symptomatology (including bloody diarrhea and kidney failure) in humans, produce Shiga toxins. These are referred to as Enterohemorrhagic Escherichia Coli (EHECs).
The recent, devastating outbreak of a virulent strain of E. coli O104:H4 (STEC O104:H4) in Europe has resulted in widespread fear and unfounded accusations against an assortment of food product manufacturers. At the time of this writing, the outbreak had sickened more than 3,000 people and was one of the worst foodborne illness outbreaks in history.
It has been learned, however, that the Hamburg strain most likely resulted from a genetic recombination of two distinct types of E. coli (one of which only occurs in humans). This recombination (or mutation) resulted in a highly pathogenic strain never before seen.