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So far, more than 20 people are dead and hundreds are seriously ill with a nasty kidney disease.

From the probable source of the outbreak, in northern Germany, the virulent  E. coli O104:H4 strain has spread to more than a dozen countries.

Now the public and doctors alike are wondering how such a common – and normally manageable – bacterium could have mutated into this deadly strain: a drug-resistant superbug.

 Escherichia coli is by no means a rare or new species of bacterium. Found in the gut of warm-blooded animals, from chickens to pigs, cattle and people, E. coli and humans have coexisted for millennia.

Indeed, you probably have about ten billion  E. coli microbes in your gut, a seething mass of a dozen strains. And mostly all are harmless; part of the body’s natural ecosystem.

E. coli is generally a problem only if it gets to where it shouldn’t be – say, into the urinary tract – or, as in this case, when a new strain emerges that is highly infectious and virulent.

The bacterium has, of course, caused problems before, notably in 1996 when an outbreak of E. coli O157 killed seven and hospitalised several hundred people in Scotland.