More than 1,800 people worldwide have been infected by highly toxic E.coli, with almost 200 new cases in Germany in the last few days.
Twelve counties are now reporting cases of the bug, which experts say is a very rare but virulent strain.
Some 18 people have been confirmed dead, with 17 of those in Germany, where the outbreak began, according to the World Health Organisation (WHO).
Many more patients are in hospital, with several needing intensive care, including dialysis.
Around one in three of those affected have been hit by haemolytic uraemic syndrome (HUS) - a deadly complication of E.coli which affects the blood, kidneys and, in severe cases, the central nervous system.
Seven people in the UK - three Britons and four German nationals - are being treated, including three for HUS.
All are from or have visited northern Germany, where experts are working to try to find the source of the outbreak.
Scientists believe salad vegetables and leaves may be the source, although WHO says the origin "of the outbreak still remains unknown".
The bug has now been identified in people in the Czech Republic, France and the United States, as well as Germany, Austria, Denmark, the Netherlands, Spain, Sweden, Switzerland and the UK.
Experts in Germany have said there are signs that the outbreak could be slowing but have warned more cases will arise.
It is already the third-largest outbreak involving E.coli in recent world history - and the deadliest.
In 1996, 12 people died during a Japanese outbreak, while seven died in a Canadian outbreak in 2000.
Last night, experts from WHO said the strain of E.coli was extremely rare and, although seen in humans previously, it has never been at the centre of an outbreak.
The strain is known to be resistant to many antibiotics, making treatment difficult.
Experts have been warning people to follow good hygiene, including washing hands after using the toilet and before touching food, to avoid spreading E.coli.
The Health Protection Agency (HPA) in the UK has issued a warning urging people travelling to Germany to avoid eating raw tomatoes, cucumbers and leafy salad, including lettuce.
It has urged anyone returning from Germany with an illness, including bloody diarrhoea, to seek medical attention.
Analysis of the bug at the Beijing Genomics Institute (BGI) showed the bacterium is an enterohemorrhagic E.coli (EHEC) O104 strain, but is a "new serotype - not previously involved in any E.coli outbreaks".
The most severe E.coli cases are usually seen in children and the elderly, but female adults appear to be the worst affected this time.
One possibility is that they eat more salad and fresh vegetables than men and children.