U.S. health officials are investigating the outbreak of the bacteria listeria that began in May. Nine people have died and about 50 have been hospitalized.

Listeria poisoning is caused by a particularly resilient type of bacteria that can survive and grow even during refrigeration, according to the Centers for Disease Control and Prevention.

Here’s what you need to know:

How can listeria get into deli meat?

Listeria bacteria thrive in moist environments, including soil and water and decaying vegetation and are carried by some animals. The hardy germs are typically spread when food is harvested, processed, transported or stored in places that are contaminated with the bacteria. When the bacteria get into a food processing plant, they can be tough to eradicate.

Many of the people in the outbreak reported eating meats sliced at grocery store deli counters. During the investigation, listeria was detected in an unopened loaf of Boar’s Head liverwurst at a Maryland store; the Agriculture Department said further testing showed the same strain was causing illnesses in people.

How does listeria make people sick?

People are sickened with listeria poisoning when they eat foods contaminated with the bacteria. Symptoms can be mild and include fever, muscle aches, nausea, vomiting and diarrhea. More serious illness can include headache, stiff neck, confusion, loss of balance and convulsions.

Listeria poisoning is tricky, because symptoms can start quickly, within a few hours or days after eating contaminated food. But they also can take weeks or up to three months to show up.

Those most vulnerable to getting sick include the very young, people older than 65 and those with weakened immune systems or who are pregnant.

Does cooking kill listeria?

Listeria can survive and grow in food even when it’s refrigerated, but the bacteria can be killed by heating foods to “steaming hot,” or 165 degrees Fahrenheit (74 degrees Celsius), the CDC says. People who are most at risk for illness should avoid the products or heat them before eating.

Most deli meats, however, are eaten cold. Because listeria can survive under refrigeration, it’s important to clean and sanitize any surfaces, including refrigerator drawers and shelves, that may have come in contact with the products, the CDC says.

What should I do if I have the recalled deli meats?

Many of the products recalled by Boar’s Head are meats meant to be sliced at grocery store deli counters, though some prepackaged meats are included in the recall.

They include liverwurst, ham, beef salami, bologna and other products made at the company’s Jarratt, Virginia, plant. The recalled meats carry the plant’s number — EST.12612 or P-12612 — inside the USDA mark on the label. The meat was distributed to stores nationwide, as well as to the Cayman Islands, the Dominican Republic, Mexico and Panama.

Consumers should not eat the recalled meats and should discard them or return them to the store for a refund.