Listeria is a gram-positive, rod-shaped bacterium that is ubiquitous and can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions. 

Listeriosis is one of the most important bacterial infections worldwide that arises mainly from the consumption of contaminated food. The disease is caused by Listeria monocytogenes, which is considered an opportunistic pathogen that affects mainly those with underlying immune conditions, such as pregnant women, neonates, and elders, resulting in septicemia, meningitis, and/or meningoencephalitis. 

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.” The danger posed by the risk of Listeria in RTE meats 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. The amount of time from infection to the onset of symptoms—typically referred to as the incubation period—can vary to a significant degree. According to the CDC, symptoms of Listeria infection can develop at any time from the same day of exposure to 70 days after eating contaminated food.