You get listeria by eating contaminated food. Unlike many foodborne bacteria, listeria can grow in your refrigerator, which is what makes it unusually dangerous. The bacterium lives naturally in soil, water, and decaying vegetation, and it enters the food supply during harvesting, processing, packaging, or storage. About 1,250 cases occur in the United States each year, but the infection is severe: nearly every person diagnosed with invasive listeriosis is hospitalized, and roughly 1 in 5 cases is fatal.
Where Listeria Lives Before It Reaches Your Food
Listeria monocytogenes is widespread in the natural environment. It thrives in soil, water, sewage, and rotting plant material, and animals can carry it without appearing sick. From these reservoirs, the bacteria make their way into food at almost any point in the supply chain. Raw materials, water, soil tracked into a facility, and even incoming air can introduce listeria to processing and packaging environments.
Once listeria colonizes a surface in a food production facility, it can be remarkably hard to eliminate. It forms protective layers on equipment, drains, and walls that resist standard cleaning. This is why a single contaminated factory can produce tainted products for weeks or months before an outbreak is detected.
Foods Most Commonly Linked to Outbreaks
Listeria outbreaks have been traced to a wide variety of foods, but they share a common thread: most are eaten without further cooking. CDC outbreak records include deli meats and cheeses sliced at store counters, soft cheeses like brie, camembert, queso fresco, and cotija, ready-to-eat meat and poultry products, hot dogs, ice cream, leafy greens, enoki mushrooms, peaches, nectarines, plums, prepared pasta meals, and even supplement shakes.
Deli counters deserve special attention. Mechanical slicers are a well-documented source of cross-contamination. A CDC study of retail delis found that in more than half the locations surveyed, at least one slicer was not fully disassembled and cleaned within the minimum four-hour window required by FDA food safety codes. Every slice of meat or cheese that passes through a contaminated slicer picks up bacteria, meaning a single dirty machine can affect dozens of customers.
Soft cheeses made with unpasteurized (raw) milk are another frequent culprit. Pasteurization kills listeria, so cheeses made from pasteurized milk and handled properly afterward carry far less risk.
Why Your Refrigerator Doesn’t Protect You
Most bacteria slow down or stop growing at refrigerator temperatures. Listeria is different. It can multiply at temperatures as low as -1.5°C (about 29°F), well below the setting of a typical home fridge. Growth is slower in the cold than at the bacterium’s preferred range of 30 to 37°C (86 to 99°F), but given enough time, listeria populations in refrigerated food can climb to dangerous levels.
This matters most for foods that sit in the fridge for days or weeks: leftover deli meats, opened packages of smoked salmon, soft cheeses, prepared salads. The longer these foods are stored, the higher the potential bacterial load, even if they smell and look perfectly fine.
Who Is Most Vulnerable
Healthy adults who ingest listeria often experience nothing more than a brief bout of diarrhea, nausea, or fever that resolves within one to three days. The real danger is invasive listeriosis, where the bacteria cross from the gut into the bloodstream and can reach the brain or, in pregnant women, the placenta. Certain groups face dramatically higher risk:
- Pregnant women are about 20 times more likely to develop listeriosis than other healthy adults. The infection can cause miscarriage, stillbirth, or life-threatening infection in the newborn. Nearly 25% of pregnancy-associated cases result in fetal loss or death of the newborn.
- People with weakened immune systems from cancer treatment, organ transplants, steroid therapy, or other conditions face severe risk. People living with HIV are at least 300 times more likely to become seriously ill than someone with a normally functioning immune system.
- Adults 65 and older have naturally declining immune defenses that make it harder to contain the infection once it takes hold.
- Newborns can acquire the infection from their mothers during pregnancy or delivery.
How Quickly Symptoms Appear
The timeline depends on which form of the illness develops. Mild intestinal symptoms like diarrhea and stomach cramps typically start within 24 hours of eating contaminated food and clear up in one to three days. Invasive listeriosis, the more dangerous form, usually takes about two weeks to produce symptoms, though the window can stretch longer. This long incubation period makes it difficult to trace the source, because you may not connect your illness to a food you ate weeks earlier.
Invasive symptoms include high fever, severe headache, stiff neck, confusion, loss of balance, and convulsions. In pregnant women, the initial signs may feel like a mild flu, with fever, muscle aches, and fatigue, but the consequences for the fetus can be severe even when the mother’s symptoms seem minor.
How to Reduce Your Risk
Cooking kills listeria. Heating food to an internal temperature of 165°F (74°C) destroys the bacteria, which is why health agencies recommend that high-risk groups reheat deli meats and hot dogs until they are steaming before eating them. For people outside those risk groups, this step is less commonly practiced but still effective.
Beyond cooking temperature, the most practical steps focus on limiting how long ready-to-eat foods stay in your refrigerator. Use opened packages of deli meat within three to five days. Keep your refrigerator at 40°F (4°C) or below, which won’t stop listeria from growing but will slow it. Clean up spills from deli products immediately, since the liquid that pools in packages of hot dogs or lunch meat can harbor high bacterial counts and spread to other surfaces.
Wash all fruits and vegetables under running water before eating, even those with rinds or skins you plan to remove, because a knife can drag bacteria from the surface into the flesh. Scrub firm produce like melons with a clean brush. Keep raw meat separate from ready-to-eat foods, and wash hands, knives, and cutting boards thoroughly after handling raw animal products.
For people who are pregnant or immunocompromised, avoiding the highest-risk foods entirely is the most reliable protection. That means skipping soft cheeses made from unpasteurized milk, refrigerated smoked seafood, store-made deli salads, and deli-sliced meats unless they are heated thoroughly before eating.