The outbreak of illness last week due to bacterial contamination of bagged spinach is one of the larger episodes of its kind, with at least 94 victims, including one death, in 20 states.

But the outbreak, which health authorities linked to spinach sold by a company in the Salinas Valley in California, is not an isolated one, says New York Times writer Henry Fountain. In the past decade there have been eight others tied to E. coli contamination of fresh greens from the valley, where most of the nation’s lettuce and spinach is grown.

Fountain asked Dean Cliver, a professor of food safety at the University of California at Davis, why these outbreaks keep occuring. Cliver said that no one, including the FDA, had been able to pinpoint sources of bacterial contamination.

The most likely source, he said, is irrigation and processing water that has been contaminated by animal waste. But poor sanitation for field workers and use of compost containing manure are other possibilities.

Dr. Cliver said greens are a quick crop, and fields are turned and replanted long before an outbreak occurs. Even if the contamination could be traced to a particular field, determining the source of the water can be difficult.