From the current study it appears the relatively high incidence of STEC O157 human infections in the UK results from the emergence and expansion of a lineage I PT21/28 clade in the last 25 years, producing strains containing both Stx2a and Stx2c prophages that are capable of higher excretion levels from cattle (supershedding) and can cause severe disease in humans. The gene discussed is STX2; the disease is infection.