Federal authorities are urging people who bought chopped romaine lettuce in the United States to throw it away because it could get them sick.

The Centers for Disease Control and Prevention said Wednesday the nationwide E. coli outbreak linked to romaine lettuce from Yuma, Ariz., has expanded to 53 cases in 16 states.

The CDC warning is clear: All store-bought chopped romaine lettuce, including salads and mixes with romaine, should not be eaten — and thrown away. If someone has eaten the lettuce and not gotten sick, it should still be thrown away. If you don't know if the lettuce you're eating is romaine or not, toss it. Eating at a restaurant? Ask the waiter if the romaine lettuce is coming from the Yuma area. If the source can't be confirmed, do not order or eat it.

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Restaurants and retailers should not sell chopped romaine from the Yuma area, the CDC said. They should also ask suppliers where the lettuce is from.

The CDC added 18 cases since the last update on Friday. The outbreak, which started March 13, has resulted in 31 hospitalizations, which includes five people developing a type of kidney failure. There have been no deaths.

Individual states, the CDC and the U.S. Food and Drug Administration are investigating the outbreak.

Pennsylvania is the hardest-hit state with 12 cases, followed by Idaho with 10. New Jersey, Montana and Arizona are among the other states affected. About 70% of those sick are women or girls.

The E. coli spreading through the states is "toxin-producing," the CDC states — specifically a toxin known as Shiga. People get sick within two to eight days of swallowing the germ, which causes diarrhea, stomach cramps and vomiting. Although most recover in one week, it could lead to kidney failure.

To avoid E. coli, wash your hands regularly and thoroughly, cook meat completely, wash fruits and vegetables, avoid raw milk and don't prepare food when you're sick. If you find yourself sick, write down what you've eaten, contact your doctor and report your illness to your local health department.