A recent listeria outbreak linked to enoki mushrooms has sickened more than three dozen people from 17 states with four reported deaths, according to federal health officials. In a food safety alert, the Centers for Disease Control and Prevention (CDC) advised that people not eat any mushrooms from Sun Hong Foods, Inc. and that pregnant women, adults over 65 and those with compromised immune systems avoid eating any enoki mushrooms labeled as “Product of Korea.”

So what is listeria and how do you know if you've been infected? Read on to learn the facts about this rare but deadly bacteria.

What is listeria?

Listeria monocytogenes is a hardy bacterium found in soil, water and animal feces. Raw fruits and vegetables and anything grown in the ground can be contaminated from the soil or from manure that was used as fertilizer. It can be found in unpasteurized milk or foods made from unpasteurized milk. The bacteria can also contaminate meat and some processed foods like soft cheeses and hot dogs and deli meats that are contaminated after being processed, says Mayo Clinic. Unlike many other types of bacteria, listeria likes the cold, making it hard to get rid of once it has contaminated an area. Listeria was also found in ice cream in a 2015 outbreak that led to three deaths and in cantaloupes in a 2011 outbreak that led to 30 deaths.

Who is at risk from listeria?

While many people may become infected with listeria, most won't develop any symptoms. Listeriosis, the illness caused by bacteria, is generally most dangerous to adult over 65, pregnant women and people with compromised immune systems, like those who have diabetes or who are taking chemotherapy drugs. In the U.S., an estimated 1,600 people become seriously ill with the infection each year; about one in five die.

What are the symptoms of listeriosis?

Symptoms are similar to the stomach flu, including nausea, muscle aches, chills and a high fever. It can also cause diarrhea and other types of gastrointestinal upset.

How is a listeria infection treated?

Most people with mild symptoms don't need treatment. More serious infections can be treated with antibiotics.

What is listeria prevented?

Follow safe food handling guidelines. Cook food properly, scrub raw fruits and vegetables thoroughly and rinse well before eating, and eat any leftovers within a few days.

Editor's note: This story has been updated with new information since it was published in April 2015.

What is listeria?
A listeria infection is a foodborne illness that can be very serious for pregnant women, those over 65 and people with weak immune systems.