It's also found in soil, water, and dust. Listeria is a hardy bacteria with lots of opportunities to spread. It can continue to grow in freezing temperatures, proliferating in sealed packages or on the walls of refrigerators. At the upper end, it survives limited exposure to temperatures of up to 170 degrees. The good news is that enough heat does kill it.
Nobody knows for sure.
Food safety experts say it's possible the products were contaminated after cooking and before packaging. Listeria usually does not pose a serious threat to most healthy people, causing only mild gastrointestinal rumblings. However, the listeria bacteria is passed on to the fetus, often resulting in miscarriage or stillbirth. Thoroughly reheating food can help kill any bacteria that might be present. If you cannot reheat these foods, do not eat them.