A never-before seen strain of bacteria is suspected to be linked to the E.coli outbreak that has hit Germany and is starting to affect other countries.
So far in the UK there has been seven reported cases of people affected by the virulent strain - four German nationals and three Britons.
Early tests indicate that two separate E.coli bacteria have mutated to form the strain.
Hilde Kruse, a food safety expert at the WHO, told the Associated Press: “This is a unique strain that has never been isolated from patients before.”
The new strain has “various characteristics that make it more virulent and toxin-producing”, she added.
While it is not unknown for bacteria to progressively evolve and exchange genes, the new strain seems to be more virulent than other strains of E.coli.
Severe E.coli cases are usually seen in children and the elderly, but all age groups are currently affected.
According to the Health Protection Agency (HPA), all the UK cases caught the infection in Germany.
Three of them have developed the potentially deadly complication of haemolytic uraemic syndrome (HUS).
According to WHO, cases of HUS and enterohemorrhagic E.coli (EHEC) continue to rise in Germany, and more than 10 countries worldwide have now reported cases.