You get listeria by eating contaminated food. Unlike many foodborne illnesses tied to undercooked meat, listeria infections often come from foods people assume are safe: deli meats, soft cheeses, pre-washed salads, and even ice cream. The bacterium is unusually dangerous because it thrives at refrigerator temperatures, meaning cold storage doesn’t protect you the way it does with most other germs.
About 1,250 people in the United States get sick from listeria each year. While that number is small compared to other foodborne illnesses, listeria is far more likely to be fatal, making it worth understanding exactly how exposure happens.
Foods Most Likely to Carry Listeria
Past U.S. outbreaks have been traced to a wide range of foods: deli meats, hot dogs, soft cheeses (especially queso fresco and other Hispanic-style cheeses made from unpasteurized milk), raw or smoked fish, raw milk, ice cream, and fresh produce like lettuce and cantaloupe. The common thread isn’t one type of food. It’s foods that are eaten without further cooking after they leave a processing facility or retail counter.
Deli meats and ready-to-eat products are particularly risky because the bacteria can transfer from contaminated equipment. Meat slicers at deli counters are a well-documented source. Listeria survives on stainless steel surfaces, and each pass of the blade can spread bacteria from one batch of meat to the next. Uncured products like oven-roasted turkey tend to support more bacterial growth than cured meats like salami, which contain preservatives that slow it down.
Raw, unpasteurized milk is another significant source. Pasteurization, the process of heating milk to a specific temperature for a set time, kills listeria reliably. Raw milk skips that step entirely, and soft cheeses made from unpasteurized milk carry the same risk. Hard, aged cheeses are generally safer because their low moisture content makes it harder for the bacteria to survive.
How Produce Gets Contaminated
Listeria lives naturally in soil, water, and decaying plant material. It’s widespread in the environment, which is why raw fruits and vegetables can carry it even when they’ve never been near a meat product. The primary route is irrigation water. When fields are watered with contaminated sources, the bacteria lands directly on the edible parts of the plant. A study of lettuce grown in Spain found significantly higher levels of listeria on crops irrigated with wastewater compared to those watered from clean groundwater.
What makes this harder to control is persistence. Listeria can survive in soil long enough to recontaminate crops throughout an entire growing season, either from the water itself or from soil that acts as a reservoir. This is why outbreaks have been linked to bagged salads, frozen vegetables, and fresh fruit, all products where the contamination happened well before the food reached your kitchen.
Why Refrigeration Doesn’t Help
Most bacteria slow down or stop growing in the cold. Listeria doesn’t. It continues to multiply at refrigerator temperatures (around 4°C or 40°F), which is what makes it unusual among foodborne pathogens. A package of deli turkey that picks up a small amount of listeria at the store counter can harbor significantly more bacteria by the time you eat it a week later. This is why shelf life matters more with listeria risk than with most other germs.
Listeria also forms biofilms, sticky colonies that adhere to surfaces in food processing plants, drains, and equipment. These biofilms are difficult to remove with standard cleaning and can reintroduce the bacteria into food products over and over. Large-scale outbreaks, like those involving ice cream or packaged salads, are often traced back to persistent contamination in a single facility.
Cooking Temperatures That Kill Listeria
Heat does destroy listeria, but it takes more than a quick warm-up. Research on low-temperature cooking found that listeria survived up to 90 minutes at 60°C (140°F) and up to 60 minutes at 65°C (149°F). For conventional cooking, reaching an internal temperature of 74°C (165°F) kills the bacteria effectively. This is the standard recommendation for reheating deli meats and hot dogs, both of which should be steamed or heated until they’re visibly steaming before you eat them.
The key point: if you’re in a high-risk group, simply warming food isn’t enough. It needs to be heated thoroughly all the way through.
Less Common Ways People Get Infected
Food is by far the primary route, but there are others. Direct contact with infected livestock or contaminated farm environments can occasionally transmit listeria to humans. These cases are rare and typically cause localized skin infections rather than the serious systemic illness associated with eating contaminated food.
Pregnant women can transmit the infection to their unborn baby through the placenta or during delivery through an infected birth canal. This vertical transmission is one of the most dangerous aspects of listeria. The perinatal fatality rate sits around 20 to 30 percent, and infection during pregnancy can lead to miscarriage, stillbirth, or a newborn with serious complications including meningitis and sepsis.
Who Faces the Greatest Risk
Listeria disproportionately affects three groups: pregnant women, adults over 65, and people with weakened immune systems. Pregnant women are roughly 18 times more likely to develop listeriosis than the general population. About 1 in 25,000 pregnant women in the U.S. are infected each year, and 1 in 4 of those infections results in pregnancy loss or the death of the newborn shortly after birth. More than half of all listeria infections nationally occur in people 65 and older.
For healthy adults with normal immune function, exposure to small amounts of listeria usually causes nothing or, at worst, a brief bout of gastroenteritis. The mild form has a median incubation period of about 24 hours, with symptoms appearing anywhere from 6 hours to 10 days after eating contaminated food.
Invasive listeriosis, the severe form that reaches the bloodstream or brain, has a much longer and more variable timeline. The median incubation period is 8 days, but it ranges from 1 to 67 days. Pregnancy-associated cases take the longest to develop, with a median of about 28 days. This long delay makes it notoriously difficult to trace the source of infection, since the contaminated food was often eaten weeks before symptoms appear.
Practical Steps to Reduce Your Risk
The most effective precautions target the specific foods and situations where listeria is most likely to be present:
- Deli meats and hot dogs: Heat them until steaming (165°F internal temperature) before eating, especially if you’re pregnant or immunocompromised.
- Soft cheeses: Choose only those made with pasteurized milk. Check the label, particularly on queso fresco, brie, camembert, and feta.
- Raw milk: Avoid it entirely if you’re in a high-risk group.
- Smoked seafood: Refrigerated smoked fish (often labeled “nova” or “lox”) carries risk unless it’s cooked into a dish. Canned or shelf-stable versions are safer.
- Refrigerator hygiene: Clean up spills from deli products immediately. Listeria can spread to other foods in your fridge and continue growing at cold temperatures.
- Shelf life: Eat perishable ready-to-eat foods quickly. The longer they sit, even properly refrigerated, the more time listeria has to multiply.
Fresh produce should be washed thoroughly under running water, though washing reduces rather than eliminates risk. Cutting into an unwashed melon, for instance, can push surface bacteria into the flesh, which is how cantaloupe outbreaks have occurred.