This article on explains in fairly understandable terms the manner in which viruses are involved in turning a strain of E. coli pathogenic.   Most strains of E. coli are not pathogenic to humans.   Some, including E. coli O157:H7, can cause serious human illness.   Shiga-toxins produced by E. coli O157 can cause hemolytic uremic syndrome (HUS) especially in the young and the elderly.