Listeria can survive in a variety of locations. A common location it is found is in soil, where it can spread to crops and animals.
“Listeria has been found in hot dogs, deli meats, pasteurized or unpasteurized milk, cheeses (particularly soft-ripened cheeses like feta, Brie, Camembert, blue-veined, or Mexican-style queso blanco), raw and cooked poultry, raw meats, ice cream, raw vegetables, and raw and smoked fish”
Although cooking or pasteurizing will kill off Listeria, food can still be contaminated with this bacteria before packaging, thus workers must follow strict sanitation guidelines to prevent it.
References:
Picture: http://www.nature.com/nrmicro/journal/v7/n9/images/nrmicro2171-f1.jpg
Listeria monocytogenes in multiple habitats and host populations [Foodborne Pathog Dis. 2006] – PubMed – NCBI. (n.d.). National Center for Biotechnology Information. Retrieved April 18, 2013, from http://www.ncbi.nlm.nih.gov/pubmed/17199514
Listeria 
