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What is Listeria?

Tricia Christensen
Updated May 17, 2024
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Listeria is a genus of bacteria most species of which, if consumed, can make people seriously ill. It can be found in soft cheeses made from unpasteurized milk, raw meat, unpasteurized milk, unwashed vegetables, deli meats, and hot dogs or sausages that are not fully heated, even if they are fully cooked before packaging. In most cases, slight exposure will not result in symptoms; however, those with lower immune systems are at greater risk for contraction.

A Listeria infection may present with fever, soreness of the neck, and vomiting or diarrhea. Complicated forms can lead to the development of both encephalitis and certain forms of meningitis. Women who are pregnant who contract illness from this bacteria can have sudden stillbirths.

Further, pregnant women are about 20 times more likely than the rest of the general population to get listeriosis. If miscarriage or stillbirth does not result, newborns can be born with Listeria infections, which can lead to higher rates of infant mortality. Since pregnant women are particularly prone to this bacteria, and the effects can be quite devastating, women who are pregnant or nursing newborns should report any symptoms of gastrointestinal illness accompanied by headaches or a sore neck and fever.

Others at risk for complications from Listeria ingestion include those with HIV, those who have received organ transplants, those with diabetes, newborns, and the elderly. It is most important that people in these higher risk groups observe some precautionary procedures that can help reduce the risk of contraction.

Those at risk should avoid raw milk or cheese made with raw milk. Deli meats should be avoided as well. If eating hot dogs or sausages, these should be fully cooked since heating destroys the bacteria cells. Any raw vegetables or fruit should be thoroughly washed. Raw meat should be avoided completely.

Recent studies show that premade salads, with lettuce, dressing and other additions that one compiles at home have also shown a higher incidence of Listeria. Though these salads may be convenient, the US Centers for Disease Control (CDC) now recommends that lettuce and other vegetables be washed, even if the label advertises these products as prewashed.

The presence of Listeria infection can be confirmed by blood test. Physicians usually ask for a fecal sample as well. When diagnosed, this bacteria is treated with antibiotics which will kill the cells and generally prevent the complications of meningitis, encephalitis and stillbirth.

In the US, when cases of Listeria are reported to a physician, the physician reports this information to both the Food and Drug Administration (FDA) and the CDC. This is helpful to identify the source of infection and warn others regarding the possibility of exposure to the bacteria. If a person is aware that he or she has been exposed to Listeria, he or she should keep in mind that symptoms can occur anytime within two months of exposure. It is advisable to see a physician and mention exposure.

Most people who contract this bacteria will not go on to have future complications. Additionally, only a small number of people are infected each year. Should someone feel he or she is experiencing symptoms common to Listeria infections, however, it is prudent to see a physician immediately, as treatment is simply a course of antibiotics.

WiseGEEK is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen , Writer
With a Literature degree from Sonoma State University and years of experience as a WiseGEEK contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.

Discussion Comments

By anon324643 — On Mar 11, 2013

Ugh, I think I have this infection. No more raw milk for me.

By anon264853 — On Apr 30, 2012

Why don't symptoms of listeriosis manifest for a few weeks?

By CellMania — On Sep 05, 2010

@dinoleash: It could be three to four weeks after consuming the contaminated food before you display any signs or symptoms of Listeria. The general rule is anywhere from one week to ninety days.

By DinoLeash — On Sep 05, 2010

How soon after eating the food would you display listeria symptoms?

By chrisinbama — On Sep 05, 2010

@cmsmith10: They are generally safe to eat right out of the pack. Nine out of ten times, they will be fine. However, it’s a good practice to heat them up anyway. Foods like that should be heated until the internal temperature is 165 degrees.

There is no way to tell that Listeria is growing just by looking at the hot dog. It doesn’t change the color, flavor, look, or smell of it. The only to be 100% safe is to cook it anyway.

By cmsmith10 — On Sep 05, 2010

So, it's not safe to eat hog dogs right out of the pack?

Tricia Christensen

Tricia Christensen


With a Literature degree from Sonoma State University and years of experience as a WiseGEEK contributor, Tricia...
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