"Michael Pollan, a journalism professor who has written extensively about food safety, said all the recent problems stemmed from the intensive, industrial-style agriculture increasingly practised in the United States.
The 0157:H7 strain of E.coli was unknown until cattle were taken from their traditional grazing grounds, pushed into industrial feedlots and fed grain and corn instead.
Faeces from those animals, in turn, can sometimes enter the water supply on big farms and find its way into irrigation canals in vegetable fields.
Professor Pollan recently likened many US farms to giant "petri dishes" in which all sorts of bacterial cultures could grow and thrive. Fast-food chains have taken steps to safeguard their ground beef - which can easily get mixed with faeces - but vegetables represent a new front in the fight for public health."