In the last decade, lettuce and spinach grown in California’s central coast region have caused at least nine outbreaks of illness associated with E. coli bacteria. Today, fresh produce outpaces even meat as a source of food-borne illness. The beef industry tightened its safety practices after Jack In the Box burgers contaminated with E. coli killed four children in 1993, but with vegetables, regulators and growers are still catching up.

As health officials have urged Americans to eat more green, leafy vegetables, the produce industry has responded to consumers’ unrelenting demand for convenience by giving them salad that’s pre-washed and packaged in plastic. Yet the convenience may have a price: Some steps in processing might actually contribute to the spread of contamination.

Even after one of the biggest food-safety investigations in U.S. history, officials can only guess at what exactly caused the recent outbreak involving bagged fresh spinach, which killed three people and sickened 201 in 26 states and Canada. Worse, they still can’t guarantee that every salad will be safe to eat.