If you’re pregnant, odds are good that someone has told you not to eat deli meat. Why? Listeria. Listeria monocytogenes is a bacterium commonly found in soil, water, and some animals. When food contaminated by Listeria is consumed, it can cause listeriosis, a rare but potentially serious illness. But why is everyone so obsessed with deli meat? Well, it’s because Listeria can live and thrive in the cold, which sets it apart from most other bugs that cause food borne illness.