Frustration is building in Europe as the source of Germany's E. coli outbreak remains unsolved. US health experts say tracking down a deadly germ can be a complicated task.

When public health officials notice an unusual number of people coming down with food poisoning, the first thing they try to do is to find out what they all ate recently. E. coli usually takes three or four days to make a person sick, but it may take a week or more. And that presents the first problem.

"Can you tell me what you ate for dinner on May 30?" David Weber is a professor of epidemiology at the University of North Carolina at Chapel Hill. He says people often do not remember everything they ate over the past week. And even if they do, they may not know all the ingredients, or where they came from. For example, he says, a tomato condiment called salsa was one of the suspects in a 2008 Salmonella outbreak in the United States.

"It's not just the salsa, but in the salsa are tomatoes, onions, peppers, spices. And then, which manufacturer made the salsa? Where did they get the tomatoes from? Who was the wholesaler? How many different growers provided tomatoes during that week to that wholesaler?"

Tracing the germ all the way back to the grower may help investigators figure out what went wrong and how to keep it from happening again. But whatever crop is causing the outbreak was picked some time ago. And by the time investigators get to the farm, "That crop may be gone. It may even be plowed under," says Ben Chapman, a food safety expert at North Carolina State University. He notes that investigators have been focusing on fresh produce, but at this point, weeks after the outbreak started, they may not even be able to trace the offending item as far as the patient's refrigerator.

"It's pretty unlikely that you have a cucumber or a tomato or lettuce sticking around in your fridge six or seven weeks after being exposed," says Chapman.

Given all the complications, it may be surprising that investigators ever figure out the cause of an outbreak. But the science of epidemiology has gotten very good in recent years, and most of the time they do.

William Keene is a senior epidemiologist for the US state of Oregon's public health department, recognized as one of the nation's best. But even he admits that there are some that get away.

"There have been a couple where there's been, really, at the end of the day, no idea what the product was. It's clearly gone. The outbreak ends. It's very frustrating. But, whatever it was, it disappeared."

Meanwhile, European officials say the number of new E. coli cases appears to be slowing down. That may mean the outbreak will soon come to an end, with or without identifying the cause.

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（来源：VOA 编辑：崔旭燕）