American retailers sell almost 6 million bags of prepared salad daily.

Over the course of three days last September, 11 cases of E. coli poisoning hit Minneapolis hospitals. No one was certain of the cause until a public health investigation was launched.

Although common culprits are undercooked meats, particularly ground beef, in this case the source of the outbreak was prewashed bagged salad.

Unfortunately, it is not known at this time how the E. coli gets into the lettuce.