Listeria is found in soil and water. It can contaminate fruits and vegetables from these sources. Animals, in turn, eat contaminated vegetables and pass the bacteria along in their meat or milk.

The high temperatures of cooking or pasteurization should kill these bacteria. Yet sometimes food can become contaminated after cooking, but before packaging. This can happen with foods such as hot dogs or deli meats.

Getting sick from listeria is extremely unusual. Certain groups of people are at high risk to become sick from listeria. Anyone with a weakened immune system is at risk. Examples include people with HIV and people with active cancer.

Pregnant women are especially at risk from listeria infections. They get sick 20 times more often than other healthy adults. Infants and the elderly are also somewhat more likely to get listeriosis.

If you've worried about something you ate or drank, you don't need tests or treatment unless you become ill or are in a high-risk group. Listeriosis can be detected with a blood test and treated with antibiotics.

Listeria is a very uncommon disease, fortunately. But following these precautions can lessen your risk:

• Thoroughly cook meat.
• Wash raw vegetables before eating them.
• Keep uncooked meat separate from cooked and prepared foods.
• Don't eat or drink unpasteurized dairy products.
• Wash your hands, utensils and cutting boards after they have touched uncooked foods.

If you are in a high risk group for listeria infection, then you should take these additional precautions.

• Avoid deli meats, luncheon meat or hot dogs unless they are cooked to steaming hot.
• Do not eat soft cheeses such as Brie, Camembert, blue-veined cheeses and others unless the label clearly says they were made from pasteurized milk.
• Do not eat pâtés or meat spreads. Canned meat spreads or pâtés are OK.
• Do not eat refrigerated, smoked seafood unless it is cooked.