Listeria can be spread to people by several different methods.
Listeria monocytogenes is found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer. Animals can carry the bacteria without appearing ill and meat or dairy products from these animals can be contaminated. The bacteria also have been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in processed food, that become contaminated after processing, such as cheese and cold cuts at the deli counter. Unpasteurized (raw) milk or foods made from raw milk may contain the bacteria.
Attached you will find an article on how to prevent and control Listeria in Food Processing environments - as well as additional articles that make for a good read.