Listeria monocytogenes, although an uncommon cause of illness in the general population, is an important pathogen in pregnant patients, neonates, elderly individuals, and immunocompromised individuals. Patients with cancer, particularly those of blood, are also at high risk for listeria infection.  See the image below.
It is typically a food-borne organism. Listeria is also a common veterinary pathogen, being associated with abortion and encephalitis in sheep and cattle. It can be isolated from soil, water, and decaying vegetation.
The most common clinical manifestation is diarrhea. A mild presentation of fever, nausea, vomiting, and diarrhea may resemble a gastrointestinal illness.  The microorganism has gained recognition because of its association with epidemic gastroenteritis. In 1997, an outbreak of noninvasive gastroenteritis occurred in 2 schools in northern Italy, involving more than 1500 children and adults. 
Bacteremia and meningitis are more serious manifestations of disease that can affect individuals at high risk. Unless recognized and treated, Listeria infections can result in significant morbidity and mortality.
L monocytogenes is a motile, non–spore-forming, gram-positive bacillus that has aerobic and facultatively anaerobic characteristics. It grows best at neutral to slightly alkaline pH and is capable of growth at a wide range of temperatures, from 1-45°C. It is beta-hemolytic and has a blue-green sheen on blood-free agar. It exhibits characteristic tumbling motility when viewed with light microscopy and is difficult to isolate in mixed cultures. It may be mistaken for streptococci or contaminants such as corynebacteria.
Most infections occur after oral ingestion, with access to the systemic circulation after intestinal penetration. Protection against Listeria is mediated via lymphokine activation of T cells on macrophages and by interleukin-18.
CNS infection may manifest as meningitis, meningoencephalitis, or abscess. Endocarditis is another possible presentation. Localized infection may manifest as septic arthritis, osteomyelitis, and, rarely, pneumonia.
The frequency of L monocytogenes infection is 9.7 cases per million population. Annually, 2500 cases are reported, with higher incidence rates during the summer months.  Pregnant women account for 27% of all cases, and most occur during the third trimester. Seventy percent of all nonperinatal infections occur in immunocompromised patients. Corticosteroid therapy is the most important predisposing association in patients who are not pregnant. Other risk factors include advanced age and recent chemotherapy.
The overall mortality rate of L monocytogenes infection is 20-30%.
Of all pregnancy-related cases, 22% resulted in fetal loss or neonatal death, but mothers usually survive.
With the exception of pregnant women, no sex predilection is recognized.
Women of childbearing age are commonly affected.
Neonates and elderly individuals are at risk.