This 2002 electron microscope image made available by the Centers for Disease Control and Prevention shows a Listeria monocytogenes bacterium, responsible for the food-borne illness listeriosis. (Elizabeth White/CDC/The Associated Press)

A pregnant Minnesota woman was among 28 people who became sick as part of a nationwide listeria outbreak, which has also killed two people in other states. So what is listeria and what’s important to know about it?

Listeria is a bacteria that can contaminate many types of food, and when eaten it can cause an infection called listeriosis. Health officials have focused on deli meat as one source of the current outbreak.

“Many people in this outbreak are reporting eating meats that they had sliced at deli counters,” the Centers for Disease Control and Prevention said in a public notice on Friday. “Investigators are collecting information to determine the specific products that may be contaminated. Products sold at the deli, especially those sliced or prepared at the deli, can be contaminated with Listeria. Listeria spreads easily among deli equipment, surfaces, hands and food.”

It’s most likely to sicken pregnant people and their newborns, adults 65 and over, and people with weakened immune systems, according to the CDC website. Others can be infected with it but they rarely fall seriously ill. Pregnant people who become sick with listeriosis can miscarry or have a premature birth, and it can cause serious illness or death in newborn babies.

The Minnesota woman who had listeriosis gave birth to a healthy baby, according to Amy Barrett, a spokeswoman for the Minnesota Department of Health. The woman was hospitalized for several days and has recovered.

As recommended by the CDC, those who are at higher risk for listeriosis should:

Avoid eating meat that’s sliced at a deli, or should heat the deli meat until it’s steaming hot or at an internal temperature of 165 degrees Fahrenheit.
Clean your refrigerator, containers and surfaces that may have touched sliced deli meats.
Call your health care provider if you have symptoms, which usually include a fever, muscle aches and tiredness, according to the CDC.

New York has had the most confirmed cases so far, with seven. The other states affected by the listeria outbreak have been primarily along the East Coast and in the Midwest, including Minnesota, Wisconsin, Missouri and Illinois.

The number of sick people is likely higher than 28 because some recover without medical care and are not tested for it, the CDC says. A national investigation into the outbreak is ongoing. More information is available at the CDC’s website at cdc.gov/listeria.