The E. coli outbreak in Germany is a new form of the bacterium, researchers and public health experts believe.
The infection can cause the deadly complication - haemolytic-uraemic syndrome (HUS) - affecting the blood and kidneys.
More than 1,500 people have been infected and 18 have died: 17 in Germany and one in Sweden.
In the UK, three British nationals have been infected - all had visited Germany.
The World Health Organisation said the variant had "never been seen in an outbreak situation before."
Scientists at the Beijing Genomics Institute in China are also reported as saying the new form was "highly infectious and toxic."
The Health Protection Agency said it was likely to be a new variant of the rare strain O104 - possibly with a newly acquired ability to infect large numbers of people.
In a statement it said: "While there is a lot more that we need to learn about this bacterium, the evidence that is already available tells us that the German authorities have been dealing with something new."
Professor Gad Frankel, from Imperial College London, the Sanger Institute and the Medical Research Council, said: "This is a new combination and a deadly combination.
"It has a gene which produces a toxin and another which helps the bacterium colonise the gut more efficiently, which effectively means even more toxin is produced.
"Research we published last year showed this same factor mediates attachment to the human gut and the surface of salad leaves."