Listeria outbreaks can be deadly. Here’s what to know about them.

Listeria outbreaks are especially dangerous for people who are elderly or pregnant, according to the Centers for Disease Control and Prevention.

The bacteria is tied to lettuces, improperly stored lettuces, meats, cheeses and other foods, according to the agency. A recent outbreak has killed one person, sickened more than a dozen others and resulted in a lost pregnancy.

People most at risk are those who are pregnant, over the age of 64, immunocompromised or newborn.

Here's what listeria is and what's going on with the outbreak:

Listeria is short for "listeria monocytogenes," the name of the bacteria that causes problems in humans. Listeriosis is the sickness that makes people ill and can be fatal.

Every year about 1,600 people get sick from listeria and about 260 people die, the CDC reports. Listeriosis ranks third on the list of food-borne illnesses that cause death in the United States.

Symptoms of a severe case usually start within two weeks after eating contaminated food — but symptoms could present anywhere between the same day or 10 weeks after ingesting the bacteria, according to the agency.

Certain foods are more likely to contain listeria, reports the CDC. They include:

Officials are looking into whether there was a single source — potentially a slicer or a distribution plant — of the listeria.

Sometimes a single source can be blamed for an outbreak, said Catherine Donnelly, a professor emeritus at the University of Vermont.

Donnelly, who has spent her nearly four-decade career studying listeria, said this is a typical outbreak.

She said the bacteria is special and stubborn because it grows on foods even when they're inside a refrigerator.

Refrigeration makes food less hospitable to bacteria and spoilage, but listeria can grow in even extreme cold if food is left long enough, which is why it's important to respect the dates on food packaging, Donnelly said.

Listeria isn't just on meat and cheese, she said. It's also found on raw vegetables and fresh fruits.

"Those of us with functional immune systems are being constantly exposed [without knowing it]," she said.

People with listeriosis report having flu-like symptoms along with headache, stiff neck, confusion, loss of balance and convulsions, CDC officials said.

The CDC recommends that people see a doctor or health-care provider about food-borne illness if they have severe symptoms such as:

Staff members at the Mayo Clinic write that symptoms of a listeria infection in a newborn can be subtle but can include:

Listeria, an intracellular pathogen, can go through gut lining and into blood. Then listeria in the blood can pass the brain barrier, causing serious infections.

One-third of people who get listeriosis die, Donnelly said.

Listeria can sneak past the defenses that bodies create during pregnancy, Donnelly said: "The organism can cross the placental barrier, which is a barrier to prevent fetuses from getting infected."

Officials don't know the extent of the recent outbreak.

"The true number of sick people in this outbreak is likely higher than the number reported, and the outbreak may not be limited to the states with known illnesses," according a CDC page related to the outbreak.

What they do know is that at least six states have people sickened by listeria.

The CDC reported that New York is home to seven of the sick people. Three live in Maryland. Massachusetts and Illinois each have two. New Jersey and California each have one person who fell ill.

Samples from sick people were collected from April 17, 2021, to Sept. 29, 2022, according to the CDC. Their ages ranged from 38 to 92.

Of the seven sick people in New York, five bought sliced deli meat or cheese from at least one location of NetCost Market, which is a grocery store chain that sells international foods. The sick people from other states bought deli meats or cheeses from other delis.

NetCost did not immediately respond to The Post's request for comment Thursday.

"Investigators do not believe that NetCost Market delis are the only source of illnesses because some sick people in the outbreak did not shop at a NetCost Market. A contaminated food likely introduced the outbreak strain of listeria into delis in multiple states," according to the CDC.

NetCost temporarily closed the deli at a store in Brooklyn last year after New York health officials told the company that three people infected with listeria ate food from there, according to the Associated Press.

Donnelly said delicatessens are particularly susceptible to listeria growth.

If meat that has listeria on it is cut on the deli slicer, the machine can spread bacteria to every slice of meat and cheese afterward, she said. From there, it can introduce the bacteria to every knife or surface it touches.

"Those slicers are incredibly difficult to clean and sanitize," Donnelly said.

The CDC and health departments take clinical samples from sickened people, Donnelly said. Health officials analyze either blood or spinal fluid to see whether there's a genetic match.

If there is a match, that commonality tells them a lot. "That suggests maybe these products were made in a common food-processing facility," she said.

Yes.

The CDC identified listeria in Dole lettuce during an outbreak that killed two people and hospitalized about a dozen this year, The Washington Post reported. The company recalled products from scores of brands and said it was working with federal authorities to investigate its operations.

A 2015 listeria outbreak tied to ice cream killed three people, The Post reported at the time. That case led to a Texas grand jury charging a former chief executive of Blue Bell Creameries, Paul Kruse, with wire fraud and conspiracy in connection with an alleged scheme to cover up the outbreak. The company pleaded guilty, according to Houston Public Media, and Kruse's first case ended in mistrial after the jury could not find consensus. Kruse's defense attorney Chris Flood told the Austin American-Statesman that of the 12-person jury, 10 jurors voted for a not-guilty verdict. The newspaper reported that Flood told jurors that prosecutors were overreaching in their charges.

No, Donnelly says.

"You wouldn't have any off-odors or off flavors," she said. "There really aren't any clues that listeria is contaminating those products."

That's why the CDC recommends that those who are susceptible not eat meat or cheese from any deli counter, unless it is reheated to an internal temperature of 165 degrees Fahrenheit — or until steaming hot.

Donnelly suggests not eating anything you're concerned about, and sanitizing your fridge and whatever surfaces potentially contaminated food may have touched.

Again, she preached caution: “Listeriosis is a rare illness. But for vulnerable consumers, they’re the ones that have to be cautious here.”