BERLIN - Schools have pulled raw vegetables from menus, piles of cucumbers sit untouched on shop shelves, and farmers say they are losing millions.

As scientists scramble to find the source of an E. coli outbreak linked to raw vegetables that has killed 18 in Europe and sickened nearly 2,000, consumers are swearing off lettuce, cucumbers, and tomatoes just in case.

"Cook it or don't eat it," Hamburg kidney specialist Rolf Stahl said at a news conference Friday. "That's my personal recommendation."

Four people in the United States were apparently sickened by the food-poisoning outbreak in Europe, U.S. health officials said Friday. Three are hospitalized with a serious complication.

All four were in northern Germany in May. Although they did not stay at the same hotel or eat at the same restaurants, officials are confident that they were infected with E. coli in Germany.

Three of them - two women and a man - are hospitalized with kidney failure, a complication of E. coli that has become a hallmark of the outbreak. One of the four fell ill while on a plane to the United States.

Two other cases are being investigated in U.S. service members in Germany, said Chris Braden, of the Centers for Disease Control and Prevention.

Consumers from the northern German city of Hamburg - the epicenter of the outbreak - to Bulgaria, Spain, France, and Sweden were worried about which vegetables and fruit they could still eat and what they should avoid.

"We no longer offer cucumbers; people just won't buy them anymore," said Mehmet Tanis, a vegetable vendor at Berlin's busy weekly market in the city's Kreuzberg neighborhood, who says his weekly profit is down $1,450.

"They're completely scared to get sick - even though we always get our cucumbers from Jordan. We're also selling 80 percent less lettuce, and only half the tomatoes."

Most of those sickened say they ate vegetables beforehand. But without being able to pinpoint the source, German health authorities have issued a broad warning to stay away from all tomatoes, cucumbers, and lettuce.

Hamburg officials initially suspected cucumbers from Spain after three samples tested positive for E. coli, but later tests showed they were infected with a different strain of the bacterium than the one behind the outbreak.

Nevertheless, the jitters have devastated the Spanish produce industry.

In Almeria, one of Spain's main agricultural regions and probably the hardest hit, the head of a farmers' association said the market for exports to the rest of Europe was still virtually dead.

Before the crisis, Almeria exported 20,000 tons of produce a day. Now many farmers are simply destroying their crops - often right in the fields where they grow - because of lack of demand and to avoid the cost of shipping them to special centers that grind unwanted crops for compost or animal feed, said Francisco Vargas, head of the Almeria chapter of the Spanish farm association Asaja.

With farmers across Europe complaining of millions in losses, Chancellor Angela Merkel agreed with Spanish Prime Minister Jose Luis Rodriguez Zapatero in a telephone conversation late Thursday to push for European Union help for those affected.