Since 1911, scientists have known it infects animals, and in 1929 the first case of human infection was detected. What is new is the recognition that L. monocytogenes bacteria may be spread in food. In earlier time, many believed farm animals transmitted L. monocytogenes to farm workers. But when listeriosis appeared in city dwellers, public health authorities realized that animal contact was not always the source of disease transmission. It has been only in the past decade that researchers have recognized L. monocytogenes as an agent of foodborne illness.
Fecal contamination is one way the organism is spread to raw agricultural products.
For example, farm animals may pick it up from consuming improperly fermented silage, and then vegetables may become contaminated when animal manure carrying the organism is used for fertilizer. L. monocytogenes is a remarkably tough organism. It resists heat, salt, nitrite and acidity much better than many organisms. Listeriosis is the disease caused by the bacteria L. monocytogenes. Consumers most commonly contract listeriosis by eating food contaminated with the organisms.