Germany is at the center of the outbreak. But with the exact source of the illness still a mystery, consumers appear to be increasingly nervous around the world.

Some 1,624 people in Europe and the United States have so far become ill, probably from eating contaminated vegetables and salads. The WHO said the strain was a rare one, seen in humans before, but never in this kind of outbreak.

The death toll is expected to rise further when Germany updates numbers later in the day after a public holiday on Thursday. Experts say the source is likely to be in Germany.

People have also become ill in Austria, Denmark, France, Netherlands, Norway, Spain, Sweden and Switzerland, Britain and United States.

"All these cases except two are in people who reside in or had recently visited northern Germany during the incubation period for the infection -- typically 3 to 4 days post-exposure -- or in one case, had contact with a visitor from northern Germany," the WHO said in a statement.

E. coli bacteria themselves are harmless. But the strain that is making people sick in Europe has the ability to stick to intestinal walls where it pumps out toxins, sometimes causing severe bloody diarrhea and other complications.

Robert Tauxe of the U.S. Centers for Disease Control and Prevention, which has been working with German health officials since last week, said the strain was likely the most deadly yet in terms of the number of deaths recorded.

"I believe it is," he told Reuters when asked about that possibility. He said it was unclear how the bacteria became so resistant.

TRADE ROW

The outbreak has put strains on trade relations, with Russia drawing EU criticism after banning raw vegetable imports from Europe and accusing Brussels of failing to handle the crisis.

German officials originally blamed the infections on contaminated cucumbers imported from Spain but later backtracked and apologized to Madrid.

E.coli infections can spread from person to person but only by what is known as the fecal-oral route. Health experts in Germany have recommended consumers avoid eating raw vegetables.

The outbreak is causing bad infections and in a number of cases, complications affecting blood and kidneys. Hemolytic uremic syndrome (HUS), which frequently leads to kidney failure and can kill, has been diagnosed in hundreds of the cases.

Many patients have been hospitalized, with several needing intensive care, including dialysis due to kidney complications.

The strain is part of a class of bacteria known as Shiga toxin-producing Escherichia coli or STEC that produces a poison known as the shiga toxin.

"The immediate public health problem is the identification of the source of infection so that it can be controlled," said Robert Hall, an expert on communicable disease control in Victoria, Australia.

He said this is done with a combination of epidemiological, microbiological and environmental investigations. "These are all highly skilled tasks that need to be done rapidly and are nearly always done in a glare of publicity," he said.

TRADE AND DIPLOMACY

Russia's ban has prompted cries of protest from European governments and a rebuke from the European Commission which urged Russia to end its ban immediately.

EU countries exported 594 million euros ($853 million) worth of vegetables to Russia last year while EU imports of vegetables from Russia were just 29 million euros, EU data show.

In Moscow, shops prepared to dump EU vegetables and consumers expressed a mixture of scorn and pride at the ban. But some disagreed strongly, saying the threat was exaggerated.

"I am not afraid of buying vegetables from any country here," said pensioner Vyacheslav Yegorov, carrying a shopping basket filled with grapes and fresh vegetables. "This thing will blow over and be forgotten tomorrow."

In Germany, some consumers were worried the disease could even spread by human contact. A church event in Dresden, attended by 120,000 people, was not serving raw vegetables altogether on Thursday, according to ZDF television.

"I noticed that there were no raw vegetables, which I found calming..." one attendee was quoted as saying.

Another participant added: "I've thought about what I can eat and what I can risk. Yesterday I noticed someone saying: yuk, there's lettuce on top of this."