(CNN) -- A virulent strain of E. coli that has killed 18 people in Europe has not affected produce in the United States, according to food safety and health officials who conceded Friday there's much to learn about the organism.
Even as efforts continue to determine what caused the outbreak, cucumbers, tomatoes and lettuce imported from Germany and Spain will be tested and cleared before they are sold in the United States, according to David Elder of the Food and Drug Administration, and the results will be shared with the European Union.
"I want to emphasize that this outbreak has not affected the U.S.," Elder told reporters in a conference call. "Produce remains safe, and there is no reason for Americans to alter where they shop, where they buy or what they eat."
Two adult women and an adult man who traveled last month to northern Germany remain hospitalized in the United States with hemolytic uremic syndrome -- a form of kidney failure -- said Chris Braden of the U.S. Centers for Disease Control and Prevention. Those three cases were reported in Massachusetts, Michigan and Wisconsin. A fourth person developed bloody diarrhea, but was not hospitalized, he added.
Two U.S. service members in Germany also developed diarrhea, Braden said. "We have no expectation that this will spread in our country."
The E. coli infection has spread to 12 countries and is blamed for at least 18 deaths -- all but one reported in Germany. About 1,800 people have been sickened.
The European Food Safety Alert Network initially said enterohemorrhagic E. coli (EHEC), a strain of E. coli, was found in organic cucumbers originating from Spain, packaged in Germany and distributed to various countries. But authorities have said that the source of the contamination has not been pinpointed.
The Robert Koch Institute, Germany's federal unit responsible for disease control and prevention, has advised German consumers not to eat raw tomatoes, cucumbers and lettuce.
Germany and Spain account for a small fraction of produce that is sold in the United States in a given year.
Officials said tracking the source of the illness could be difficult. If it, in fact, stems from the produce, the tomatoes, cucumbers and lettuce could have been eaten together in a salad.
"We don't know yet what is the cause and each of these (vegetables), individually or perhaps in some combination, appear as likely culprits, so I don't think we know enough to hypothesize at what point in the supply chain contamination may have occurred," said Don Kraemer of the U.S. Food and Drug Administration. "We don't know which one that actually had the contamination in it."
The CDC is warning any U.S. citizens who have recently traveled to Germany to seek immediate medical care if they begin to show symptoms of the bacterial infection.
Braden said officials were unable to identify the source for an outbreak of this E. coli strain in the republic of Georgia in 2009.
The strain is more common among women and adults, he said, and officials are not certain how it may be associated with produce. "We have a lot to learn about this particular organism," Braden said.
The officials told reporters the U.S. Food Safety Modernization Act signed by President Barack Obama this year provides steps that minimize the risk of E. coli at U.S. farms and production facilities.
"We believe prevention is superior to responding to an outbreak such as this one," Kraemer said.