Listeria bacteria are found widely in the environment in soil, including in decaying vegetation and water, and may be part of the fecal flora of many mammals, including healthy human adults. According to the FDA, “studies suggest that 1-10% of humans may be intestinal carriers of Listeria.” Another authority notes that the “organism has been isolated from the stool of approximately 5% of healthy adults.” Overall, seasonal trends show a notable peak in total Listeria cases and related deaths from July through October.
Ingested by mouth, Listeria is among the most virulent foodborne pathogens, with up to 20% of clinical infections resulting in death. These bacteria primarily cause severe illness and death in persons with immature or compromised immune systems. Consequently, most healthy adults can be exposed to Listeria with little to any risk of infection and illness.
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. One study 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.
The prevalence of Listeria in ready-to-eat meats has not proven difficult to explain. 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 favor 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.