How does listeria get into food?

Soil. Listeria lives in soil and decaying vegetable matter where our food is grown. It can be passed to meats and cheeses from vegetables, and it is able to grow and multiply at refrigerator temperature. Hot cooked foods do not harbor listeria.
Widespread. Listeria is ubiquitous, which means it can be found in soil, water, and the intestines of many mammals including humans. It is easy then to see how it finds its way into food. Fortunately most people do not develop disease when they ingest it. It usually affects pregnant women and the immunocompromised.