The death toll rose to 17, with German authorities reporting that an 84-year-old woman with the complication had died on Sunday.
Medical authorities appeared no closer to discovering either the source of the infection or the mystery at the heart of the outbreak: why the unusual strain of the E. coli bacteria appears to be causing so many cases of hemolytic uremic syndrome, which attacks the kidneys and can cause seizures, strokes and comas.
"This particular strain we're dealing with now seems to be unique," said Dr. Hilde Kruse, program manager for food safety at WHO Europe:
Germany's national health agency said 1,534 people in the country had been infected by EHEC, a particularly deadly strain of the common bacteria found in the digestive systems of cows, humans and other mammals. The Robert Koch Institute had reported 1,169 a day earlier.
The outbreak has hit at least nine European countries but virtually all of the sick people either live in Germany or recently traveled there.
The Robert Koch Institute said 470 people in Germany were suffering from hemolytic uremic syndrome, or HUS, a number that independent experts called unprecedented in modern medical history. HUS normally occurs in 10 percent of EHEC infections, meaning the number seen in Germany could be expected in an outbreak three times the size being currently reported.
That discrepancy could indicate that a vast number of cases haven't been reported because their symptoms are relatively mild, medical experts said.
But they also offered another, more disturbing theory -- the strain of EHEC causing the outbreak in Europe could be more dangerous than any previously seen.
"There may well be a great number of asymptomatic cases out there that we're missing. This could be a much bigger outbreak than we realize right now," said Paul Hunter, a professor of health protection at the University of East Anglia in England. "There might also be something genetically different about this particular strain of E. coli that makes it more virulent."
There are hundreds of different E. coli strains in the environment -- every person naturally carries the bacteria -- but only a very small percentage are dangerous.
German Agriculture Minister Ilse Aigner said scientists were working nonstop to find the source of the germ that is believed to have been spread in Europe on tainted vegetables -- and where in the long journey from farm to grocery store the contamination occurred.
"Hundreds of tests have been done and the responsible agencies ... have determined that most of the patients who have been sickened ate cucumbers, tomatoes and leaf lettuce and primarily in northern Germany," Aigner said on ARD television. "The states that have conducted the tests must now follow back the delivery path to see how the cucumbers, or tomatoes or lettuce got here."
German authorities initially pointed to cucumbers from Spain after people in Hamburg fell ill after eating fresh produce. After tests of some 250 samples of vegetables from around the city, only the three cucumbers from Spain and one other of unknown origin tested positive for E. coli.
But further tests showed that those vegetables, while contaminated, did not cause the outbreak. Officials are still warning all Germans to avoid eating raw cucumbers, tomatoes or lettuce.
Some experts said it might be impossible to ever identify what caused the outbreak, as much of the tainted fresh produce may already have disappeared from markets.
"As in many foodborne disease outbreaks, the culprit may never be identified and the epidemic just fades away," said Brendan Wren, professor of pathogen molecular biology at the London School of Hygiene and Tropical Medicine.
To identify which E. coli strain is responsible, scientists must grow the suspect bacteria in a laboratory, which can take up to two days. Once that's done, tests to characterize the strain may take another day or two and those tests can only be done in specialized labs.
"These are complicated molecular tests and it's not something you can do in one day," Hunter said.
In the Czech city of Brno, lab workers wearing white coats and latex gloves meticulously snipped hundreds of pieces off of cucumbers, peppers and other vegetables from Spain, Germany, the Netherlands and Denmark as well as eastern and central Europe to be tested before allowing them into the Czech market for sale.
"None of the samples has tested for the bacteria so far," said lab chief Pavel Alexa.
"It all takes up to four days to do the basic research to determine if a sample is suspicious," Alexa said. If the sample is suspicious, it takes days more to isolate the strain of E.coli responsible, he said.
Spanish officials said, meantime, that they were considering legal action after Europeans swore off Spanish produce in droves after the initial report. And in Germany, farmers' association president Gerd Sonnleitner said the call for people to avoid raw vegetables had cost local farmers an estimated euro30 million ($43 million) so far.
Germany typically sees a maximum of 50 to 60 annual cases of HUS, which has up to a 5 percent fatality rate according to the World Health Organization.
More than 60 percent of the EHEC cases in Germany have been women -- 88 percent over the age of 20 -- and nearly 90 percent of the HUS cases have been women over the age of 20.
Experts suspect the outbreak may be mainly striking women because they are the ones most likely to be eating fresh produce.
It remains unclear why most of those affected are adults and not children or the elderly, who are normally more susceptible to this illness, said Kruse, the food safety expert at the WHO.
"We should be open to whatever the investigation shows, but adult women are more likely to be exposed to vegetables than other populations," she said.
Last week, Reinhard Burger, head of the Robert Koch Institute, said it was also possible more women were affected because they were predominantly the ones handling food in the kitchen.
The World Health Organization said cases of the E. coli illness have been reported in nine European countries: Austria, Denmark, Germany, the Netherlands, Norway, Spain, Sweden, Switzerland and the U.K. All but two cases are either people in Germany, or people who had recently traveled to northern Germany, the organization said.
U.S. health officials say two people who had recently traveled to Hamburg and are now back in the United States have the bug. The agency did not say where in the U.S. the two travelers are, but said it is working with state health departments to learn more about the cases and identify others.
An American tourist who traveled to the Czech Republic from Germany was hospitalized in Prague, officials said.
In addition, Sweden has reported 15 cases of HUS, followed by Denmark with seven, the Netherlands with three, the U.K. with two and Spain with one, according to the European Center forc Disease Prevention and Control.
The Dutch Institute for Public Health and the Environment said that the five people in the Netherlands all recently visited Germany and four have serious kidney problems as well as stomach complaints.
It's "extraordinary" to see so many cases of the kidney complication from a foodborne illness, said Dr. Robert Tauxe, a foodborne disease expert at the U.S. Centers for Disease Control and Prevention. "There has not been such an outbreak before that we know of in the history of public health."
He added that the strain of E. coli in the European outbreak has not been seen in the United States, where there have been several high-profile foodborne outbreaks in recent years, but none with such a high death toll.
There's little precedent in Europe, either. In 1996, an E. coli outbreak in the United Kingdom caused 216 cases and 11 deaths.
The World Health Organization said 86 percent of those sickened in the current outbreak were adults, and two-thirds were women. It said it was unusual that more children weren't affected.
Kirsten Grieshaber and Juergen Baetz in Berlin, Karl Ritter in Stockholm and Jan Olsen in Copenhagen contributed to this report.