While state and federal officials have traced the current outbreak to a California company's fresh spinach, they haven't pinpointed the sources of the bacteria. The regulatory agency does not consider the contamination deliberate.

"There is always a question in the back of our mind whether it may have been a deliberate attack on the food supply. Currently, there is nothing in the epidemiology to consider this deliberate," Acheson said.

That leaves a broad range of other possible sources, including contaminated irrigation water that's been a problem in California's Salinas Valley. About 74 percent of the fresh-market spinach grown in the United States comes from California, according to the California Farm Bureau Federation.

Nineteen other food-poisoning outbreaks since 1995 have been linked to lettuce and spinach, according to the FDA. At least eight were traced to produce grown in the Salinas Valley. The outbreaks involved more than 400 cases of sickness and two deaths.

In 2004 and again in 2005, the FDA's top food safety official warned California farmers they needed to do more to increase the safety of the fresh leafy greens they grow. "In light of continuing outbreaks, it is clear that more needs to be done," the FDA's Robert Brackett wrote in a Nov. 4, 2005, letter.

Suggested actions included discarding any produce that comes into contact with floodwaters. Rivers and creeks in the Salinas watershed are known to be periodically contaminated with E. coli, Brackett said.

Various produce growers' associations worked with the FDA to publish new guidelines for the safe handling of spinach and other leafy greens in April after the agency reiterated its concerns.

The spokesman for a group representing 3,000 growers and shippers in California and Arizona said the new guidelines were not directly in response to any particular incident.

"The basic standard for the industry is zero tolerance," said Tim Chelling, of Western Growers.

But one food safety analyst said the Salinas Valley was developing a reputation for food safety problems connected to leafy greens.

"Even the biggest companies have become vulnerable," said Trevor Suslow, a microbial food safety researcher at the University of California, Davis.

And with the food-safety risk comes increased vulnerability to lawsuits. In the case of Brayden Leafty in Utah, over-the-counter anti-diarrheal medications were not effective, and on the evening of Sept. 1, Brayden's stools became "grossly bloody," according to his mother's lawsuit.

The following day, Brayden was rushed to Cottonwood Hospital, where he received intravenous hydration, pain and nausea medication, the lawsuit said. The boy was taken to Primary Children's Hospital when his condition worsened on Sept. 3.