Dear Doctors: We had to throw away some sliced turkey from the deli because of a listeria recall. We had used some of it to make sandwiches, but fortunately no one in the family got sick. What is listeria? What happens when someone gets sick from it?

Dear Reader: Listeria is a bacterium that causes a serious foodborne illness known as listeriosis. About 1,600 people get sick from listeria infection each year, and about 260 of them die.

Listeria is found in soil, water, decaying vegetation and moist environments. It can also live in the digestive tracts of some animals, including cattle and poultry. Listeria can enter our food chain at multiple steps in the process. This includes during harvest, as foods are being transported, during their manufacture and preparation, and while they are stored in environments contaminated by the bacterium.

Many foodborne bacteria can’t multiply at temperatures below 40 degrees Fahrenheit, which makes refrigeration an effective food-safety mechanism. However, listeria can continue to grow at temperatures as low as 24 degrees F, well below freezing. It’s not until the environment reaches temperatures of 0 degrees F (or below) that listeria will stop multiplying.

The most common way to become infected is by eating food that has been contaminated with bacteria. Possible sources include processed and cured deli meats, hot dogs, fresh fruit or vegetables, soft cheeses, cold-smoked fishery products and unpasteurized dairy products, particularly raw milk. Healthy adults who ingest the bacteria don’t always get sick.

It is most dangerous to older adults, people with weakened immune systems, pregnant people, fetuses and newborn babies. Although someone who is pregnant may have only a mild illness, the bacteria can cause severe disease in the fetus or newborn baby. Listeria infections during pregnancy can also lead to miscarriage or stillbirth.

Older adults and those who are immunocompromised are at higher risk of developing an invasive form of the disease. This can cause severe infections of the bloodstream, leading to a life-threatening condition known as sepsis. It can also cause infection of the brain and lead to meningitis or encephalitis.

When confined to the intestines, symptoms of listeriosis can begin from a few hours to a few days after infection. In invasive disease, it can take from several weeks to more than a month for symptoms to appear. For someone with intestinal disease, which tends to be milder, symptoms may include fever, nausea, vomiting, diarrhea, muscle aches and fatigue. In more severe cases, where the disease is systemic, symptoms include headache, stiff neck, loss of balance, confusion and convulsions. Diagnosis begins with a patient’s symptoms and recent history and is confirmed by a lab test known as a bacterial culture. Treatment ranges from the use of medications to manage symptoms in mild cases to antibiotics in more severe disease.

To avoid foodborne illness at home, keep the fridge clean and wipe up spills immediately. Wash hands before and after food prep, and thoroughly clean cutting boards and kitchen tools. Listeria can also be present in pet food, so take the same care with a pet’s dishes, as well.

Send your questions to askthedoctors@mednet.ucla.edu.