The Prevalence of Listeria in Food and the Environment
Listeria bacteria are common in nature.
Listeria is a common presence in nature, found widely in such places as water, soil, infected animals, human and animal feces, raw and treated sewage, leafy vegetables, effluent from poultry and meat processing facilities, decaying corn and soybeans, improperly fermented silage, and raw (unpasteurized) milk. [18, 23, 27]
Foods commonly identified as sources of Listeria infection include improperly pasteurized fluid milk, cheeses (particularly soft-ripened varieties, such as traditional Mexican cheeses, Camembert and ricotta), ice cream, raw vegetables, fermented raw-meat sausages, raw and cooked poultry, and cooked, ready-to-eat (RTE) sliced meats—often referred to as “deli meats”. [18, 21, 23, 28] One study found that, over a five-year period of testing, in multiple processing facilities, Listeria monocytogenes was isolated from 14% of 1,080 samples of smoked finfish and smoked shellfish. 
Ready-to-eats foods have been found to be a notable and consistent source of Listeria. [14, 21] For example, a research-study done by the Listeria Study Group found that Listeria monocytogenes grew from at least one food specimen in the refrigerators of 64% of persons with a confirmed Listeria infection (79 of 123 patients), and in 11% of more than 2000 food specimens collected in the study.  Moreover, 33% of refrigerators (26 of 79) contained foods that grew the same strain with which the individual had been infected, a frequency much higher than would be expected by chance.  A widely cited USDA study that reviewed the available literature also summarized that:
In samples of uncooked meat and poultry from seven countries, up to 70 percent had detectable levels of Listeria . Schuchat  found that 32 percent of the 165 culture-confirmed listeriosis cases could be attributed to eating food purchased from store delicatessen counters or soft cheeses. In Pinner  microbiologic survey of refrigerated foods specimens obtained from households with listeriosis patients, 36 percent of the beef samples and 31 percent of the poultry samples were contaminated with Listeria.
The prevalence of Listeria in ready-to-eat meats has not proven difficult to explain. [26, 29] As one expert in another much-cited article has noted:
The centralized production of prepared ready-to-eat food products…increases the risk of higher levels of contamination, since it requires that foods be stored for long periods at refrigerated temperatures that favour the growth of Listeria. During the preparation, transportation and storage of prepared foods, the organism can multiply to reach a threshold needed to cause infection. 
The danger posed by the risk of Listeria in ready-to-eat meats has prompted the USDA to declare the bacterium an adulterant in these kinds of meat products and, as a result, to adopt a zero-tolerance policy for the presence of this deadly pathogen. [7, 29]
A USDA Baseline Data Collection Program done in 1994 documented Listeria contamination on 15.0% of broiler-chicken carcasses . Subsequent USDA data collection did not test for the prevalence of Listeria in chicken or in turkeys. [31, 32]