Cucumbers, raw lettuce and tomatoes are the prime suspects.
The E. coli bacteria responsible is a new strain that has never been seen before, according to the World Health Organization
Thursday, Russia expanded its ban of produce imports from Spain and Germany to include all vegetables from the entire European Union.
Northwestern Germany is the hardest hit region, but the outbreak has spread to nine countries in Europe.
Also Thursday, German authorities withdrew their claims that imported Spanish cucumbers were to blame.
The outbreak is pointing out gaps in the U.S. food safety system, raising concern that similar outbreaks might happen here.
Though there is no reason to stop eating fresh vegetables in the United States, officials are monitoring the situation carefully. The Food and Drug Administration has stepped up testing of those foods imported from affected countries as a precaution, although very little is imported.
When it comes to fresh produce, a sweeping new law requires the FDA to set standards to guard against contamination of all sorts. The rules are expected to address such things as properly processed compost, worker hygiene, and keeping animals and their runoff from fields or irrigation water.
It's not clear how quickly those rules will emerge; Republican-led efforts to cut FDA's budget would strain the work.
The Associated Press contributed to this report.