The largest produce-related outbreak of E. coli in the US has been traced to spinach grown in California's Salinas Valley farmland. To date, at least 157 people in 23 states have fallen ill from the E. coli bacterium.

One person, a Wisconsin resident, has died. Eighty-three individuals have been hospitalized, and 27 have developed kidney failure, according to the US Centers for Disease Control and Prevention.

A confirmed positive sample from a bag of Dole baby spinach has established a link to San Juan Batista-based Natural Selection Foods, a large-scale producer of the ready-to-eat product. Health officials are now focusing their search on nine farms in California's Salinas Valley, the region where most of the spinach consumed in the US is grown. All of the affected spinach is believed to come from that vicinity.

"The likelihood that we will get it back to a specific farm is good because of the number of cases and because of the UPC codes on the packages," Dr. David Acheson, chief medical officer of the US Food and Drug Administration's Center for Food Safety and Applied Nutrition, told reporters on Wednesday.

But he said the chance of discovering the precise cause of the contamination would be small.

Ground water is one possibility, as almost all of the the water used for irrigation in the Salinas Valley comes from uninspected private wells. Agricultural practices associated with watering and fertilizing crops, maintaining sanitation and controlling pests are based on a set of voluntary rules.

A previous E. coli outbreak associated with spinach occurred in the Salinas Valley last year, Acheson noted. He acknowledged that a greater effort should have been mounted at that time to prevent a recurrence. The current outbreak is the largest caused by fresh produce in the US, in terms of number of victims.

States affected: Arizona, California, Colorado, Connecticut, Idaho, Illinois, Indiana, Kentucky, Maine, Michigan, Minnesota, Nevada, Nebraska, New Mexico, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington, Wisconsin and Wyoming.

More cases have been reported in Wisconsin than in any other state -- 40 so far.

Natural Selection Foods announced a recall of its prepackaged spinach and salad products containing spinach on Saturday.

Several other companies that use spinach supplied by Natural Selection in their salad brands, including River Ranch Fresh Foods and RLB Food Distributors, also have recalled their products.

The FDA is advising consumers to discard any packaged fresh spinach they may have on hand and not to buy any brand of the product until further notice. Washing the spinach does not remove the bacteria.

E. coli bacteria infect approximately 73,000 people a year in the US, causing an average of 61 deaths annually, according to the CDC.

They normally reside in animals' intestines and enter the human food chain through fecal contamination. Health officials stress the importance of thoroughly washing all raw vegetables. Other foods that may contain E. coli include raw milk, unpasteurized juices, water and undercooked or raw meat -- particularly ground beef.