When can I start eating bagged romaine again?

Lettuce has a short shelf life and a lot of retailers have taken bagged romaine off shelves. “Hopefully with it being in one particular growing region and that region moving to California, it won’t be too much longer,” said Laura Gieraltowski, who leads the C.D.C.’s food-borne outbreak response team. That said, she urged consumers to wait for the all-clear from the C.D.C. before eating chopped romaine.

“It’s a fast-moving outbreak,” she said. “We’re getting reports of new illness daily from our state and local health departments.”

Why is this outbreak so worrisome?

Escherichia coli is in our intestines, the environment, foods and animals, but most of the time it doesn’t make you sick. However, this particular strain — Shiga toxin-producing E. coli 0157:H7 — is particularly dangerous. Symptoms appear from one to 10 days after eating and can vary, but may include severe stomach cramps, bloody diarrhea and vomiting. While the people affected range in age from 10 to 85 years, the median age is 34 — meaning the bug is making otherwise healthy, strong people really sick. The hospitalization rate for E. coli illness is typically around 30 percent, but this strain has put 67 percent of the cases in the hospital. Five people have developed hemolytic uremic syndrome, a type of kidney failure. Given delays in reporting, those numbers are expected to grow.

The other concern is that unlike a recent multistate outbreak of salmonella in eggs, which resulted in a recall of a specific egg product, no specific grower, supplier or brand has been identified as the source of the romaine outbreak.

Where has the outbreak occurred?

Infections linked to the outbreak have been found in 16 states, illustrating how widely bags of romaine are distributed around the country. You can find a list of states where cases have been reported on the C.D.C. website. Most of the reported cases have come from Pennsylvania (12), Idaho (10), New Jersey (7), Montana (6) and Arizona (3). New York, Connecticut, Ohio and Michigan have had two cases each, with one case each in Alaska, California, Illinois, Louisiana, Missouri, Virginia and Washington state.

How do I know if the greens I’m eating contain romaine?

Most people can’t reliably distinguish romaine from other forms of lettuce, Ms. Halloran said. A head of romaine lettuce is more oblong than the round shape typical of iceberg. “It’s the one with the pointy football shape,” Ms. Halloran said. “When it’s chopped up you’re not going to see that. It’s a bit crunchier and chewier than bibb lettuce. The outer leaves are dark green and the inner leaves are yellow so color isn’t going to tell you. If you’ve had Caesar salad you’ve almost certainly had romaine lettuce.”

Why does the C.D.C. say it’s O.K. to eat heads and hearts of romaine, but not bagged?

The documented cases have been linked to bagged romaine eaten at home or in restaurants, not full heads or hearts of lettuce. The lettuce at the source of the outbreak is grown in Arizona for bagged use. The C.D.C. suggests that the contamination is limited to some part of the bagged lettuce supply chain.